Set the cutter to it's slowest setting, then once you get things cutting good you can step up the speed if you want although I've gotten to where I typically run mine pretty slow. I'm in no rush and would rather it cut good than fast.
I also run only 60 degree blades. I know they say it's over kill for simple sign vinyl but the cost is minimal and it's just one less thing to mess with.
The offset will vary from blade to blade. Typically blades from the same manufacturer and degree will have the same offset, but you'll want to pay attention when you change blades. The test cut from the control panel of the cutter is not affected by the offset in the software. You'll need to draw your own test image, a box works well, and do test cuts from the software.
Once you get all that dialed in, if it's still not cutting the GreenStar stuff as well as you think it should, put some Oracal in and give it a try. You will be surprised how much nicer that stuff cuts. You may need to adjust your blade depth. The GreenStar is slightly thicker I've found.