I use a fishing scale to pull my belts to 12kg tension, making sure all the belts are the same tension.
This removes the guesswork from belt tension, something I had problems with before I pulled out the fishing scale.

 

Without the 0.5mm spacer the dual belt will be loose, this might be part of your issue. Many people use a double sided tape with the required thickness or glue something in between, I used cheap pultruded carbon fiber 6x0.5mm strips like $4 a meter.
When I cut holes they do turn out slightly undersize, with what I do I just deal with it. If I need it to be spot on like for a bearing I will do a test cut and adjust the diameter of the cutter in the software until it's a snug fit.
Gary