My sizes are just slightly off
I did a 40x40 cube with a 30mm hole going straight through
the size of the finished item was 40x40x40 the outside was spot on but the hole was supposed to be 30mm diameter and it was 29mm diameter
how can I get this 1mm back
the print was done with a 0.8 nozzle
This is a common issue with holes in 3D prints. Selecting "External perimeters first" or "Outside-in" in the slicer helps; but you will probably have to specify the hole as being slightly larger than you actually want it.