You are right that exposure to UVB (from either direct sunlight or a proper UVB light, neither of which should pass through glass or plastic) will allow the chameleon to produce D3 which will allow it to use the calcium in its system. Sunlight is the best source of UVB and you likely don't have to add any D3 through supplements if your chameleon gets enough sunlight. (It needs to be able to move out of the sunlight though when it wants to.)

Since many of the insects used as feeders have a poor ratio of calcium to phosphorous, its important to dust the insects at most feedings with a phos.-free calcium powder to make up for it.

The use of a supplement that has D3 in it IMHO is just to make sure that the chameleon gets a base amount that will not create an overdose and the UVB light will do the rest. I dust twice a month lightly with a phos.-free calcium/D3 powder.

I also dust twice a month with a vitamin powder that has a beta carotene (prOformed)source of vitamin A. Beta carotene will not build up in the system like prEformed vitamin A will. However its not certain whether all/any chameleons can convert beta carotene or not...so some people give their chameleons a little prEformed once in a while. Be careful though...excess prEformed vitamin A can build up in the system and also interfere with the D3 and push the chameleon towards MBD.

Gutloading/feeding the insects a nutritious diet is also important. For crickets, roaches, superworms...a wide variety of greens (dandelions, kale, collards, endive, escarole, etc.) and veggies (carrots, squash, sweet red pepper, sweet potato, celery leaves, zucchini, etc.) can be used.

If you have a veiled chameleon, you can feed these same greens and veggies to it along with a small amount of fruit (apple, pear, melon, berries, etc.).

Appropriate temperatures aid in digestion so that's important too.

Calcium, D3, vitamin A and phos. are all important players in bone health and other systems and need to be in balance. When trying to attain a balance you need to look at the supplements, what you feed to the chameleon and what you feed to the insects.

Hope this helps!
BTW...I've been keeping chameleons for over 20 years.

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