It's not an exact amount, I think that's why you see so much variation between recommendations. The goal of the supplements is to enhance feeder bugs with minerals and vitamins that they typically have in the wild but don't when they are raised in captivity. Just a light even coat of the supplement on the bugs at each meal should do the trick. You can offer plain calcium most days and they some vitamins instead of the calcium once or twice a week. This becomes less frequent as they get older and start eating more veges. Salads don't need to be supplemented because they are already rich in nutrients.
Offering D3 is recommended when using lower output bulbs such as a T8 or compact UVB. With the higher output bulbs such as a T5 tube or a good mercury vapor bulb, they should produce enough UVB to allow for a dragon to synthesize D3 naturally - so plain calcium without or with low amounts of D3 is recommended.
To simplify things, you can just dust with an all-in-one option like Repashy Calcium Plus. That should be balanced enough to use regularly. Otherwise, plain calcium most days and substitute a multivitamin once or twice a week should do the trick.