Marty, I had the exact same experience when I tried to use Illustrator with my SVG file. Even when I print from Mozilla using the SVG plugin it misses the mark by about %-6% in terms of the sizing.

By the way, I did try my mask for the first time two nights ago using an Orion 80ED. It worked great in Nebulosity using the Fine Focus tool. I need to post a screen shot later today.