38% is a starting point. generally, start w/ 70% of the room width for spacing the monitors and create a 60° equilateral triangle with the listening position @ 38% from the front wall and the monitors approx up to the front wall. shrink or expand to fit based on monitor size and actual positioning available.
larger monitors typically need to be further away than small ones. you want the right balance so the monitors don't sound "too close" or "too far". if you're monitoring at 77dB-85dB this will approximate the "right" level for most situations.
monitor height about 38-42% of room height or balanced to listener height.
