How many teeth you calculate the belt to be long?

I have asked that same question before and seems there is no problem.

But personally i would try all belts to be as short as possible. Actually on the machine i am building now i had to use 1200mm belt at one place /z/ and i worked on and changed the design until my longest belt is 305 mm. Again, its absolutely personal, but i wouldn't use something longer than 350mm.