Most slicers make holes smaller than the actual design for various reasons. This is a chart you can print so you can test fit items before designing the proper holes for it. I made it only 2.5mm thick as there is no need to go thicker like some other test prints I found. Also I don't skip sizes like some others (I.E. 10, 12, 14). This is only round but I may do a square one as well latter. The holes are in 1mm increments from 1mm to 25mm. Print with settings you use most often as this will show you normal sizes for your prints. Mark with pen the holes on the print (like image included) note that the first hole (1mm) will likely be filled in and not visible but still mark it. Also look to make sure as the 2mm hole may not print open on some printers. Just remember the first row is from 1mm to 10mm so you can work backwards if needed to get the right markings.