I purchased one of these grinders as well, also my first experience with a chain grinder. The outside lines are the 10° marks. Not all 3/8 chain uses the 10° tilt, look at the guide provided in the manual or the manufactures documentation for your chain. You may also want to get the Oregon manual for their 511a grinder, this grinder is a copy of it, their manual is far better and is available in PDF format on their website.
For a quick fix on the chain stop I put some tread tape, the thin Teflon stuff you put on pipe fittings, a few wraps of that stuff and it will tighten it up enough so it doesn't vibrate around anymore and is still easily adjustable. You will have to move it a little bit to keep it centered properly when switching from one side of the chain to the other.
I have had a good time with this grinder, once setup properly and you get a few chains done it becomes very fast and easy to use and will keep your chains sharper and more consistent, it really helped straighten out some older hand filed chains.
P.S. I have been able to find all the grind angles for all manufactures of chain except Stihl, if anybody knows where I can find that info that would be great.