Well part of it is simply that I've probably been doing this a lot longer than you have :) But when you practice, landing on the right part of the thumb is a huge part of what you're practicing. Finding where to land so you don't have to adjust, and learning to land there every time, is really the key. You may do some tiny adjustments as you land, so you're also learning to do that as smoothly as possible and eventually you don't really need to adjust much at all.
As for the angle of the nail, yes, I think that's personal. I like the sound I get at, and the amount of resistance I get from the nail, at that angle. A lot of players like that 90 degree angle and that works for them. That's why I try not to say "you must do it exactly this way." I call this process "finding your thumb." You need to find what works for you, and it only works through experimentation. But once you find it life becomes much easier and everything sounds better, so it's definitely worth the time!