Afternoon Chip,
Generally speaking, at std fuel line pressure a std T3 turbo on a YB will maintain a decent AFR at up to 24psi held to the limiter. A T3 map tails the fuel off from 5k as the peak efficiency envelope has been passed with that turbo and the power drops off, so this is your danger area with too much boost. A T34 or larger will obviously drop this boost figure slightly whilst making more power. A real T34 map also wont start to lean at 5K as the peak power is now higher.

On my own T3 std YB chips i recommend the boost be set to peak at around 25 and hold around 22.This gives a good air fuel ratio margin and helps to control ACT's on the std intercoolers. Merry Xmas.