I'm no expert, but I think not just leg length but femur length more specifically.
I probably wouldn't go over 170 unless I had a high BB height for the pedal strike reason.
Spindle length - no magic formula. There are reference books to be consulted, and you can always ask here. One of the variables will be - if you're using a double crankset for your conversion, are you planning on using the inner or the outer landings? That's a difference of 5mm chainline right there.