Here we go...supplements 101...
I use a phos free calcium powder dusted lightly on all the feedings except 4 a month. Once every second week I use a phos free calcium/D3 powder dusted lightly on the insects and the opposite two weeks of the month I dust once each of those two weeks with a vitamin powder with a beta carotene source of vitamin A.
Now for the reasoning...
I've don't it for years with Yemen and Panther chameleons and others too and they live long heathy lives.
Phos, calcium, D3 and vitamin A need to be in balance.
Most feeder insects have a poor ratio of calcium to phos so I dust lightly with calcium at every feeding to make up for it.
Without D3 the chameleon can't use the calcium in its system so imdust twice a month with the powder that has D3 in it to ensure that the chameleon gets some D3 without overdosing it. D3 from supplements is fat soluble and ready to use so it can be overdosed. I allow the chameleon to use the UVB light provided for it to produce the rest of the D3 it needs. The D3 produced from UVB should not build up in the system and cause issues as long as the chameleon can move in and out of it at will.
There are two forms of vitamin A (both fat soluble)...prOformed from carotenoids) and prEformed (from retinol, retinyl, retinal, etc) ...prOformed can not build up in the system because the body converts it as needed. PrEformed can build up and lead to health issues so we need to be careful with it...which is why I use the beta carotene one...because it's safe. However...it's not known how well chameleons convert beta carotene so it might be necessary to provide a little prEformed vitamin A once in a while.
In addition to this the D3 and the Vitamin A must be in balance.
Doing the dusting as I described at the beginning seems to workso I've continued to do it this way.
Now..feeding/gutloading the inaects properly is important too..as well as appropriate temperatures and watering.
Hope this helps.