Take it off, straighten it out so it's vertical as it comes down, and tighten slowly, a few turns on one side, a few turns on the other side, holding it totally vertical, until the 3 points of contact (2 in front and one in rear, as shown in your instructions) are almost touching, like the thickness of a couple of business cards. As you get close, be careful to look at each point of contact, because one will always be a smidge closer than the other two, unless you have the thing plumbline-vertical. Be careful not to tighten if ANY points of contact ARE touching, because that's how you crack it.
Tightening down and compressing that rubber gasket is what you are supposed to do. You won't crack it if no porcelain on the tank is touching any porcelain on the base. No porcelain on porcelain contact, and you will be fine. I cant tell from your photos, but right now it doesn't look totally straight front to back. Sometimes it takes a few tries where you have to loosen it, make it vertical, and start tightening again. My first one ever took like 3 or 4 tries. After that...cake. Sometimes it helps if one person holds it and the other person tightens while checking where the points of contact are relative to the tank.
The points of contact don't have to actually "contact" the tank -- very, very close is good enough. It will be nice and stable as it sits on the compressed gasket if you get it to within the thickness of a couple of business cards. (Our contributor Jim says a couple of pieces of paper; either way.)