Originally Posted by Road Fan

the formula is TRAIL = (DIAMETER * COS (HTA) - OFFSET)/ SIN (HTA). I use millimeters for all lengths and degrees for the angle.

You measure wheel diameter with a ruler. The head tube angle is hard to measure if you don't have a good digital angle measuring device. Fork offset, well my method is kind of intricate, I'll let others chime in.

If you delve into the code this is what you should find as a kernel.

But i think the Yojimbo tool is accurate if you have accurate inputs.

I really don't know any rules of thumb or other estimates.