First off, what sort of gecko is it? (I'm going to assume leopard, dont ask, just a hunch
) Its hard to say how much calcium is too much. You cant really overdose your gecko on Ca (the excess should simply pass through), but you can overdose on D3 which is often mixed into the calcium. If you feed your crickets a gutload formula, I would still powder them but maybe half as much. I'm not a big fan of calcium sand as I've heard testimonies where it clumps and clogs the intestines etc but I would assume that the gecko would take whatever extra calcium it needs from it (this is what they do in the wild as well).
Your gecko runs away because it is probably scared. Most young leopard geckos run away because they are not familliar with you. Its your best bet to handle it but not any more than you already are since it may cause stress. As geckos grow up, they slow and tame down.
All reptiles carry salmonela in some respect. I would venture to say geckos are some of the cleanest reptiles I've dealt with so the risk should be minimal. However, I would still recommend to wash your hands after feeding and handling.