What I do is this...
I dust at most feedings with a phosphorous-free calcium powder.
Most insects have a poor ratio of calcium to phos. so this helps to make up for it.
I use a vitamin powder twice a month that has a beta carotene source of vitamin A. There is controversy about whether chameleons can convert beta carotene...so some people give them preformed vitamin A once in a while. Preformed vitamin A can build up in the system, but beta carotene can't. Excess preformed vit. A can prevent D3 from doing its job and lead to MBD.
Because my chameleons rarely get sunlight, I dust lightly with a calicum/D3 powder twice a month. Excess vitamin D3 can build up in the system too.
I gutload my crickets and other insects. Be careful to take into consideration the amounts of phosphorous, preformed vitamin A and D3 that are in anything that you use in the gutloads.
Appropriate basking temperatures aid in digestion.