Scaling not working with NC drill file

I have an inch based NC drill file for a PCB.

With a standard import into Sheetcam the scale box is greyed out but showing 25.4

The file imported correctly.

Problem is the PCB I have produced is a tiny bit undersized so I want to scale the import of the drill file to match.

Tried a custom scale number but the measurement from hole to hole did not change. Tried a much bigger difference in the scale number (20:1) and still no change in the imported file.

Any ideas.

Cheers,

Keith.