I don't understand why people would decrease the calcium dusting for a montage species...aren't the ratios of calcium to phosphorous still/always unbalanced in the insects? IMHO its the fat soluble vitamins that need to be watched...just my opinion.
This is straight from the jacksons care sheet, It's always been explained to me that because their metabolism is slower they don't require as much calcium or vitamins.
Supplementation:
Calcium and other vitamins are very important to your chameleon's health. Feeder insects should be lightly dusted with powdered supplement before being fed to your chameleon. As a montane species (native to higher altitudes) Jackson's have decreased supplementation requirements compared to tropical species due to metabolism differences. Use calcium (without D3 or phosphorus) twice a week, a multivitamin once a month, and calcium with D3 once a month.