The gauge doesn't have to be perfect, remember that you're measuring two different locations and getting the difference basically. So if the gauge is a little crooked it actually comes out in the math (which is being done by the gauge). The car doesn't have to be level, you want it at ride height- so the alignment numbers you set are the ones that you're running down the road with. The thing you don't want to do is have the car sitting differently than it does on the road, like if you use turntables in the front and leave the rear on the ground. That raises the front a couple inches, and changes your caster reading. Some guys will do their alignments with a person or weight in the front seat for the same reason, it does change things a little. That's more of a race thing, for a daily like mine the weight balance changes a little when I haul stuff, have a passenger, go to the store, etc. The floor should be level, or mostly so. The 20* measurement comes from the gauge. The best way to measure it is with the turntables. The SPC gauge has a pointer on it, you can use just the gauge to get that reading. There's more room for error doing it that way, because you're just eyeballing the pointer. But it will totally still work, you might just want to check the final alignment a couple times to make sure you get similar readings each time.
