You should use the bump stop spacing & adjust the t-bars to get it near what it's supposed to be (mfr should tell you...).
The second dimension you mention is supposed to get bigger with the UCA/T-bar lift; so the factory spec you mention no longer applies.
The real determinant is how much t-bar crank allows you to get proper caster/camber alignment numbers & I believe this occurs with ~1/2-3/4" UCA-bumpstop spacing for most/all UCA lifts available.