It really is a personal thing unfortunately BUT, you can judge your pumping in a number of ways. The pull you are talking about is good, as long as it is not too hard. If your breasts are inflating into the domes and you are feeling a pull then that is good. Your breasts can turn anywhere from a pink to a red colour however, any signs of blue or purple colouring could be a sign of overpumping, that goes for blotchiness too. Ifyou release the dome pressure and you have little dots on your boobs then you have over pumped too.
The thing is, if you overpump once or twice your boobs aren't goimg to drop off or anything, you will just know your own personal pumping range. Don't be too afraid, watch out for purple colouring, it should never hurt your breast when you pump. Unfortunately, the edges of the domes can dig in your chest a bit and that can sometimes be uncomfortable but that is just something we have to deal with if we are not using padding when using the NB.
Good luck and let us know how you get on ;D