How much room do you want at the chest?
Whatever value you provide here will simply be added to your chest circumference measurement when drafting the garment.
Given that an undershirt has no arms, a little chest ease goes a long way. Add too much, and you'll end up with gaping armholes.
You should only use negative ease if you are using a stretch material that you want to fit tightly. Overall stretch should be configured with the stretch option.