Sketch dimensions on different planes?

Hello, I´m pretty much a newbie and a student spending my time on different sites learning a lot all the time, but now I stumbled across a problem on dimension and parameters. I have two questions actually, but if you can help on either of them, I would appreciate it a lot. I´ll try to be clear what I´m after although I might not have the right names for the things, bare with me...

(I´m using Fusion 360, but the principles are propably the same in other programs also.)

I have modelled a gas tank that is round. Think of a little bit squashed egg. For learning purposes I designed my sketch so that I can easilly change the dimension in parameters. I got it working pretty good, but now I´m doing the hole for the cap and I want it to be movable/changeable (in the design) the same way as my tank dimension are. The hole is sketched on the surface so it should stay there (I hope), but if I move it to the area that has a curve on the tank surface I would like the hole to rotate on an angle driven by the circumference of a another circle and so adjusting to the tank surface. What a centence! (I´m planning on using the same sketch later on a collar for the cap) I think I would need to set a distance from the hole sketch to a location in the tank sketch. Am I making any sence here?

I can´t seem to be able to set the distance on another plane other that the one I started to sketch with.

If I can do that, then there is another question... Can I set boundaries for sketches to move to a certain distance and then keep a set distance beyond that point? In this case I want to be able to set a distance for my cap hole from the center of the tank. I can do that by setting a user parameter and then changing that the way I want. I can try different designs and be a happy camper. That´s all good on the flat part of the tank, but I want the hole to follow the outline of my tank when it starts to curve. I don´t know how to think this. Should I somehow attach the sketch to my tank surface or a path of some kind or try to set dimension that restrict and change the movement when I change my parameters?

I´m a bit lost here. Please point me into a right direction and I´ll start digging information from the internet.

Thank you.

- Jari -