contraptor belt size calculations

[TunaSandwich]

I have two questions about belts.

1) Given the contraptor 10 tooth pulleys, is the the forumla to get
from shaft distance to belt size [Shaft Distance" * 2] + 2" = belt
size. So if you need 100 inches between your shafts, then the belt
size is XL202? Or if you want 30" between shafts, then it's XL600?
Does this seem right?

2) Is buying a bunch of open ended belt a good idea? Then we could
support any contraptor shaft distance. I was thinking of this since
belt sizes are not all even. At least with SPI and McMaster, not all
shaft sizes (ones that are contraptor compatible) are supported.

Any comments?