Because the insects we use as feeders have a poor ratio of calcium to phosphorous we recommend dusting at most feedings with a phosphorous - free calcium powder to help make up for it.
It's recommended that you dust twice a month with a phosphorous - free calcium/D3 powder to ensure that the chameleon gets some D3 without overdosing it and leaving the chameleon to produce the rest of the D3 from its exposure to the UVB. Exposure to the UVB will not lead to an overdose as long as the chameleon can move in and out of the UVB when it wants to.
We also recommend dusting twice a month with a vitamin powder that contains a beta carotene (prOformed) source of vitamin A. PrOformed sources of vitamin A cannot build up in the system and lead to overdoses like prEformed sources can and this leaves it up to you to decide whether to give your chameleon some prEformed if it needs it.
Calcium, vitamin A, vitamin D3 and phosphorous are all important players in bone health and other systems in the chameleon and need to be in balance.
Appropriate temperatures are also important since they play a part in digestion thus indirectly in nutrient absorption.
Hope this helps!