Calculating thread depth for cutting threads on a lathe

Hi all, I've been here only a short time and only posted acouple of times. I know there's a lot of much smarter people here then me. I'vebeen trying to work out thread cutting on my hercus model A, the part I'mhaving trouble with is getting the right thread depth. The formula that I'veworked out is my own as I'm not a machinist, only a hobbyist.My formula in short is ( for imperial) 1000divided by the thread pitch divided by 2 multiplied by (for a 55 deg thread)the tangent of 62.5 degree =1.921 (say a thread of 36 tpi .277 divided by 2 =.138 x 1.920 would have a thread depth of .248)or for a 60 degree thread you would multiply it by 1.732.
Does that sound anywhere near the right way to calculate thethread depth?
I've tried it and it seems very close, the threads seem toknit together ok but maths isn't my strong point and this formula is one basedon angles and tangents.

Any thoughts?
Stephen