I dust at almost every feeding with Rep-cal phosphorous-free calcium. This helps make up for the poor ratio of calcium to phosphorous found in most feeder insects.
I dust twice a month lightly with Rep-cal phosphorous-free calcium/D3.this ensures the chameleon gets some D3 without overdosing it. D3 from supplements can build up in the system and lead to health issues.
I dust twice a month with Herptivite. This has a beta carotene (prOformed) source of vitamin A so it won't build up in the chameleon's system like prEformed sources can and leaves you in control of the prEformed vitamin A.
It's important to feed/gutload your insects well too. I feed/gutload crickets, superworms with a wide assortment of greens and veggies such as dandelion greens, kale, collards, endive, escarole, carrots, zucchini, squash, sweet red pepper, sweet potato, and a bit of fruit such as pear, apple, melon, berries.
It's important to provide a proper UVB light such as the long linear Repti-sun 5.0 so the chameleon can produce the D3 needed to use the calcium in its system. D3 from UVB shouldn't build up as long as the chameleon can move in and out of the light.
It's important to provide appropriate basking temperatures so that the chameleon can digest it's food properly.
Hope this helps!