Why are you set on stopping at that particular number? Generally all you do is make sure your tires aren't contacting stuff like radius rods and steering drag links when cranked all the way over. Also, if you are using an early Ford I beam they have spindle stops that hit the inside of the spindle when turned sharp, that acts as a stop. But even if you are using an aftermarket tube axle there are limits to how far the steering box will go in both directions, so that usually is what stops it from going any further.
We generally worry about caster, camber and toe in, but this is one specification I have never even given any thought to............it is a non issue.
Don