Ho do you measure belt tension? (no replies)

Hi All

I'm struggling with distorted circles on my reprappro mendel and am assuming it is a belt tension issue causing backlash (I'm also getting a gap between infil and perimeter). I'm finding it hard to find out how tight the belts should be in any quantifiable way and this leads me to ask if there is a way of quantifying belt tension? If the anser is yes how tight should the belts be?

Thanks

John