Hi Ger21, I have checked with our MDF supplier and found out that in Australia we have 18mm thickness.
The metric conversion of 3/4" to mm is 19.05. So we could machine a 25mm board down to 19.05mm, or as you suggest - change the dimensions to suit the 18mm board. Not sure which way to go yet.
Found another good thread which has a file for nested base cutting of the MDF board -
http://cnczone.com/forums/showthread.php?t=53757
Look at post no.2 from Joe.
For interested members, we will post more on this subject as progress is made.
Rocket