but if you DO want to have a go via probing there are numerous ways you can do it. A simple way (not the most optimized) is to jog your probe down into the hole roughly in the centre (but doesn't really matter). Probe west, note the X returned - NOT the current position displayed. (in Mach for example the X trip point will be in gcode var 2000 your controller/interpreter may differ). Now probe east, note the X position. G0 or G1 mid way between your recorded points. Now probe north and south and note the Y trip points (in Mach gcode var 2001) then move to the mid point of your recorded Y trips. Voila - your at the centre - or at least as accurately as your probe allows and your holes are circular. The probe command in Mach is G31.