What species and age is your cham? Montane species need a different dusting schedule. Does the repti calcium have added vitamin D3? If so, you will also need a plain calcium without D3.
For veileds and panthers, dust lightly every day with the plain calcium. Dust with the calcium D3 every 2 weeks, and use the vitamin dust once a month.
It's a veiled about 7 months. The calcium dust does have D3 and the vitamin supplement is a spray that says aids in calcium metabolism on it. Also says it has beta carotene.
The amount of D3 in the Zoo Med product is quite low, if I remember correctly. I believe you could use that twice a week, maybe 3 times a week. You'd use a vitamin free calcium and mineral powder the rest of the week. And a vitamin powder a couple times a month, depending on whats in it and how you gutload
Edit: no wait, Im not sure about it being low D3. Best to check that yourself. If its 20,000 IU per pound or more, use once a week at most. If its less D3 than that, use it more often.