Good starting spot is usually where the instrument is when sitting. Sit down, adjust strap, then stand and try it. You can adjust for personal preference from there (I normally play standing with the instrument slightly lower to accommodate what is comfortable for my right arm/hand).
Playing standing up takes practice. Some things to possibly expect if you haven't run into them already:
kink in neck form looking down at an angle your not used to
back ache from bending back/weight of instrument, etc.
instrument moving on you
left or right hands or arms not hitting things as cleanly because angle front to back or neck angle is slightly different.
Time spent experimenting with angle of the instrument and length of strap while practicing playing standing up in short sessions should cure most of these problems over time.
At least a guitar doesn't weigh near as much as a banjo.