the very first guitar i built back in 72 i did everything including cutting the fret slots for the fingerboard. How do you know where to put them? with a mathematical equation. You take your scale length and divide it with an integer, subtract what's left and divide it again. Then repeat 24 times because when you get to 12 or 24 you can verify that your math was right. This was in the days before computers or calculators. I only knew one person who had access to a calculator and he had to sneak it out of work. Now with excel you can write a simple equation in 5 minutes and have your results in seconds. The world has changed.