Belt drive

I have ordered one of the new Grizzly mini-mills and plan to make a CNC
machine out of it. I am viewing the mill as a 'part machined kit of
castings' I expect to throw away just about everything except the
castings, scrape them to a good fit and build up from there.

I intend to use belt drive between the stepper motor and the leadscrews.
Are there any tips on setting the correct pulley ratio?

Anyone have a program that calculates belt length given pulley diameters
and center distance?

Where is the best place to order belts and pulleys?

What is best to tension the belt- slotted screws for mounting or an
idler pulley?

Ive got some nema34 steppers of 250oz/in rating. Are these likely to do
this small mill?

thanks for the tips,

ron