Autonow,
The machine you have was made for engraving rather than accurate machine work so it would not surprise me that you have some errors in dimension using Newlydraw.
There are a number of different options for using Mach3, calibrating your machine and then achieving accuracy but it does not come cheap.
I don't wish to recommend any particular solution because I have not tried any of them but 'slot in boards' may be machine specific ie. if your particular machine controller does not step at exactly 1000 steps per inch they will not work accurately.
Replacing the existing controller with a new 3 axis board that is Mach3 compatible is perhaps cheap but is a radical solution and you must consider that your existing setup lets you 'print' a design from Corel etc. changing to Mach3 would mean converting that design to GCode before you could produce it.
Please remember that although Mach3 can handle some basic raster operation it's main strength is in vector work.
Hope this helps.
Tweakie.