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I am doing a classical, well somewhat of a hybrid as it's for a steel string player who wants a slightly narrower neck -1 15/16 - than a normal classical and a slight radius on the fretboard - 20 in.
What do I need to consider regarding fretboard and neck to body geometry? I understand that those who build with a slipper heel plane in a slight taper in the fretboard about 1/32 in. from nut to 12th fret.

That's a big question Ron!

You need the string height off the top at the saddle to be right (10-11mm) and the action to be right (typ. 4mm and 3mm) which generally calls for the neck to be tilted forward. How much tilt depends on how you build and how much top curvature there is and whether you want any concavities at all in the sound board. Most classical building styles result in some sort of concavity (purely cosmetic, but it's there) around the sound hole as the convexity of the top washes out to a flat upper bout.

If you build using radius dishes, you can avoid that, but it takes some cunning. This guitar:

is a pure classical that looks like a steel string. It has a double tenon neck joint like this:

I prefer to have the plane of the top of the neck coplanar with the upper bout and get the neck angle right by wedging the fretboard up at the nut end. I achieve this using a continuous, wedged, sliver of neck wood, rather than tapering the fretboard itself. It has some knock-on complications with truss-rods and the interface with the headstock, but these are relatively simple to overcome once you understand the implications of building this way. If you match the wood well, the joint between the neck wedge and the neck proper at the nut, where it is 3 - 4mm thick, disappears.

The glue line is about half way between the bottom of the fretboard and the line of reflection at the right of this photo. (No, I can't see it either!)

There are lots of different ways of achieving this sort of thing and which you chose depends a lot on your existing tooling infrastructure. I do it this way because I can use the same infrastructure (radius dishes, outside moulds, go-bars, bolt-on necks) to build both classical and steel string guitars; basically any type of "flat top" guitar without having to invest in/store heaps of different types of tooling.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thanks for the response Trevor. I too use my existing tooling for building - body molds, go-bar deck and radius forms etc. Good to know that this is in use by other builders as well. I will add this to my library of information and inch ahead.
Great looking guitars. I like the glueless heel to body joint.

I had also decided my next build will be a classical bolt-on, so this is timely!

Being my "second" build (anticipating I may actually finish my first) i have a couple naive questions:

In my limited, uninformed, kept-in-a-closet view of guitar building, I'd think i could use the same methods for a SS, keeping the radiused top. This discussion then, is just in regards to what adjustments are needed due to the lower stress on the neck from the lower string tension? I assume that means that less back angle is required? Or does a fully domed top require the same setup regardless?

Trevor... I have been procrastinating buying your books and eventually i'll get there, but when i go to the link you provided for this guitar, there's also a link to "Buy the book". Going to that page it talks about how the book(s) contain plans for 4 models of guitars. Is this classical one of them? Might be a reason to stop procrastinating...

"This discussion then, is just in regards to what adjustments are needed due to the lower stress on the neck from the lower string tension? I assume that means that less back angle is required?"

Classicals generally have a higher action than steel strings and the neck is often tilted slightly forward, rather than back. Some builders build the neck in the same plane as the soundboard and taper the fretboard to achieve the same result, as previously mentioned.

Go figure. Obviously i have no classical experience.

Is there a reason behind the higher action? Do nylon string vibrate more and therefore need more clearance to avoid buzzing?

I believe classical's strings are under less tension then typical steel strings and as a result have more oscillation or displacement for a given strumming force.

The books have 4 sets of plans; two classical, one with Fleta bracing and the other with CF lattice bracing and two steel string, one with X-bracing and one with CF falcate bracing (just to show that there is a lot more possibilities in steel string guitars than just X-bracing). The guitar shown in the first pic above is a neo-classical falcate braced cutaway built for a customer who wanted matching SS and classical guitars. Here's the pair:

The Build book largely follows the construction of a guitar exactly the same as the one on the left (the SS). The neo-classical is sort of a smaller bodied version of that, and whilst there are a lot of similarities in technique between building the two, there are lots of differences in design; neck angle being just one. The books do not have plans for the neo-classical cutaway, but it's essentially a marrying of the cutaway SS design and the classical designs. Whilst there are very detailed instructions on how to build four different guitars, the books are really about how to design and build any "flat-top" guitar, so you get exactly what you want.

The power in a string is proportional to the tension in it but also proportional to the pluck amplitude squared. So twice the pluck amplitude gives you four times the power. But you need the higher action (responsible for the difference in neck angle) to get the amplitude, but with the lower tension of classical strings it's easier to deal with. That's how classical guitars get their volume; the good ones being louder than a lot of SS guitars. All this sort of technical stuff, essential to knowing how guitars actually work, is in the Design volume.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thanks for the info Trevor!