Can you tell me what version of ArtRage you're using?
Updating tablet drivers is also always a good idea.
Switching to 'mouse mode' in ArtRage changes from using the high-precision tablet coordinate system to using the pixel-resolution mouse coordinates supplied by Windows.
I think in future versions of ArtRage I'll just give up on the precise coordinate mode, and do it the way everyone else does it. After all, if you need sub-pixel accuracy of placement of lines, you can always 'zoom in' to the canvas.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"