nanoDLP

Hello to all 3D print fan's!

I have been experimenting alot with 3D print, my models finnaly got the right shape in a relatively short printing time, but I've come to a size problem. I draw a model with 10mm wide hole but the hole varies from 7-8mm. Can anyone explain me how to correct my printer to be able to print 1:1 objects? I'm printing down-top, the projector is projecting over a mirror on the bottom part of polimer box. I thought to move the polimer box a little bit higher to make a bigger distance between my reflecting glass but I would want to be sure on how much to move it. Is there any equation or any experienced advice?

I can attach pictures of my printer just to show the position.

Cheers