That calculator matches the sheet called "Belt Length" in mine:

http://www.mycncuk.com/threads/5848-...g-belt-lengths

Looks like there's a slightly discrepancy on the sheet where I did it the other way round. Perhaps I should have worked out the formulas myself, not copied the nearest source...It's two circles and two lines, how hard can it be!

I think the problem here is more the tolerances on the pulleys and belts - they're not much, but enough to require a small amount of center distance adjustment to get the correct tension.