IMHO you should be dusting the insects with a phos.-free calcium at most feedings to help make up for the poor ratio of calcium to phos. found in most feeder insects. The ratio of the insects doesn't change...so why should the dusting not occur at almost every feeding??????
Gutloading is also important. Crickets, roaches, locusts, superworms, etc. can be gutloaded with a wide assortment of greens (dandelions, kale, collards, endive, escarole, mustard greens, etc.) and veggies (carrots, squash, celery leaves, sweet potato, sweet red pepper, zucchini, etc.)
I dust twice a month with a phos.-free calcium/D3 powder, but my chameleons are rarely outside since I live in Canada. If you have them outside often you shouldn't need to use D3 at all.
I dust twice a month with a vitamin powder that has a beta carotene source of vitamin A. Beta carotene (prOformed) vitamin A will not build up in the system like prEformed sources can but there is controversy as to whether all/any chameleons can convert the beta carotene so some people give a little prEformed every once in a while. (I never have with dwarf Jackson's or veileds, or most other chameleons)...and they live good long lives.) Excess prEformed may prevent the D3 from doing its job and push the chameleon towards MBD.