It is possible to overdose on vitamins, thats why I think twice a week is fine. just a small pinch so the crickets are lightly dusted, not swamped in the stuff. as soon as the get free in the viv (leos allowing ! the crickets start cleaning themselves.
as to the calcium, i dust the crickets twice a week also, alternate days of course to ensure the leos do get some.
We do dust all the crickets we feed to the leos on those days, not just some.
Thats 4 days supps and 3 free days, in which they have the bowl of calcium powder in the tanks, and they do seem to know when they need it.
When you think about it, mass farmed crickets are almost empty of any goodness, they provide the bulk. Thats why its recommeded to gut load crickets in addition to the supps above.
Just out of interest, has your leo just shed, and if so, has all the skin been removed. I do sometimes see one that has some left and it seems to impeed his/her movement for a while. If so, up the humidity a little in the hide, and maybe spray him a little to help.
Hope this helps
Tony