what guitar is it?
front of nut to centre of 12th fret x 2 gives you the number used to calculate fret position. This is the number almost everybody but gibson refers to as scale length and will do fine for what you want.
Strings are compensated (saddles moved back) to allow for the fact you bend the string slightly sharp when fretting, as mentioned above different strings (and set-ups!) require different amounts.
When placing a bridge i adjust the saddles about 7/8ths of the way forward and place the high E break point bang on the scale length measurement. some people set it to the middle of the bridges adjustment, which is often fine but i find it wastes the available saddle travel for setting intonation. you should never really need to move the saddles forward of the scale length - the 1/8 i leave in front is fudge room, i dont need 1/2 the bridges adjustment as fudge room! But you never know when someone will want heavier strings or a higher action so extra backwards adjustment can come in very handy
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gibson's scale length is not the number used to calculate fret position. they advertise 24 3/4" scale length but the 'nut to 12 fret x 2' number is actually closer to 24 5/8" or 24 9/16" depending on year. this often confuses people when checking the scale length on gibson's. The 24 3/4" they refer to is often called a 'compensated scale length' meaning that after compensation the string length is about 24 3/4"
its always seemed daft to me, because each string is a different length and none may actually end up being 24 3/4". the other way is just more accurate!
