When you design a tadpole how do you choose the angle of the post the crank is mounted on? Are there some sites that explain these things?

I'd aim for about 65 degrees from the line described by the power side of the chain. That's not a hard and fast number, there's wiggle room in either direction. Within reason, a shallower angle will let you shift a greater range of chainrings, and a steeper angle... won't.

If in doubt, measure the angle on some uprights.