obviously the manufacturer would have the best answer but heres where i would start:
make sure the reducer and the white ink have the same or close cure temp so your flash and cure times stay the same
what are you trying to change about the ink? climbing the squeegee, dot gain, viscosity, etc...
for example:
we have an white ink thats fairly sticky - we add some "viscosity buster" to loosen it up so it flows better through the screen but dosent change the opacity and bleed resistance
a white ink that needs to be reduced to pass though a 230 screen - we would use fashion soft base
a white ink that we use for neck tags - we dont want it to show through the back of the shirt but we want to have some body to the ink so the rapid tag machine dosent fling the ink around - we would use soft hand clear.
i would start with less reducer that what you need - on an auto press the ink will loosen up over the print run and behave slightly different on shirt 1 and on shirt 100.