It's just a flip of a switch on my manual machines DRO, doesn't seem like it should be too hard...just need 25.4 to display 1.000.
Make sure Lock DRO's is NOT checked.
Leave your Native Units where they are (mm I presume).
MDI G20 and whatever metric value you have displayed (25.4) will be converted and shown as 1.0000 (inch).
MDI G0 X10 and the machine will move to X10 (inches).
MDI G21 and that will be converted and shown as 254 mm.
MDI X 25.4 and it will move to 25.4 mm
MDI G20 and that will convert and display as 1.0000 (inch)
JOG where ever you want, MDI G20 or G21 to display the moves in Inches or MM respectively.
G20 (inch) and G21 (mm) are modal of course so be sure to put it back where you need it.
I had a button to toggle the DRO's on a mill screen but never used it much so I removed it.
If I understand your question correctly.
Regards,
Russ