I use a common truss rod which gets install 7/16" below the fretboard plane. I also like a pretty slim neck and I've gone as thin (including fretboard) as 0.790". Now I make my fretboards 7/32" thick too, so that gains me 1/32".
So, yes your formula is what I tend to use, although, you really want to know what a "standard" neck thickness is. They can range from slim 0.780" to thick 0.950" or more.
So for me, this would be the formula at the nut.
Since I know my total thickness desired and my fretboard thickness desired, along with the depth required for my trussrod, I know how much material is left in the neck.
Total-FB=neck
0.83"-0.219"=0.611"
Neck-TR slot=remaining
0.611-0.438"=0.173 which is just over 11/64 so, I have more than the desired 1/8" remaining wood.
This is not really a 'formula' per say, but rather it's just doing the math based on known criteria.
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