timing belt gear reduction advice

If I use a timing belt around two pulleys do I need to allow for
some adjustment in the motors position to compensate for slight
differences in belt length and to aid in installing and removing the
belt. Or can I simply have the motor with its pully and the shaft
with its pully at a calculated fixed distance, and stretch the
pulley around the pulleys.

I believe I can accuratly get the distance between two pullys and
the length of the belt calculated pretty accuratly using Sdp/si
little belt calculator: http://www.sdp-si.com/Cd/default.htm

Thanks
Troy