I always thought the discussions I've seen about nut slotting seem to make things much more complicated than they need be with things like the Stew Mac gauge, feeler gauges under strings, etc. I've also noticed that a large majority of repairs that have come through my shop could use a little nut work... I feel like getting nut slots the right depth is one of the most important things affecting playability yet time and again I see examples where there is a lot of room for improvement.
Anyway, I measure things with a very simple tool: a thin straight edge about 4" long... long enough to cover the first two or three frets and thin enough to fit inside each slot. First, check to see if the slots are too high... I want the slots to be exactly on the plane of the frets just like a zero fret would be. With the guitar at pitch, slide the rule up next to each string and have a look... it's usually pretty obvious if the slots are too high. This guitar's B string needs just a little tweak; it's close but could be better:
If it needs a deeper slot, loosen the string a few turns and slide it over to the next slot. File down a little with your favorite nut file, then place the rule in the slot and over the frets. If there is any daylight under the first fret you're not there yet. If it hurts your eyes to focus on such a tiny space, push down and pull the ruler back until it falls out of the slot... if it clicks that means it dropped down and hit the 1st fret meaning you're not there yet:
Repeat until you can slide the rule back in each slot without a click (or a very light one for folks who want a little extra nut action). I do this while the guitar is at tension otherwise depending on the neck, there may be some back bow which will mess up your fret plane making you go too deep. And if you do go too deep, cut a piece of paper in the shape of the nut's footprint and place it under the nut as needed. I've found that CA and bone dust stink as filler material.
I can usually tweak all six slots within 5-10 minutes and there's no math or complicated gauge/jig setups. I don't begrudge anyone their own slotting technique - there's a bazillion ways to do every task and whatever works for you is likely best for you - but I thought I would share what I think is a super simple and effective way to do this... I hope it helps!