drill bit size

one time on a turret lathe i was making a hole for a 1 1/4 bolt. i used the reg chart. but after taping it. it didnt look like the thread was deep enough. so going off the formula in the book. did the math. and used a boring bar, instead of a drill. then it looked much better. from then on i only did it by the math. a program in my pocket computer made it very easy. if it wasnt for calculators and computers i would never have been able to be a machinist.