Honestly, whilst I may be throwing somewhat of a spanner in the works here, using both synthetic supplements and plain old calcium is quite outdated and compared to what we are now capable of doing, guesswork.
Nobody actually knows how much D3 reptiles need. There just hasn't been a Pugh research to determine the answer. However, we do know full well that all reptiles (and pretty much every vertebrate, at least) makes their own vitamin D3 under UVB rays; it's the 'sun vitamin' that everyone raves about, responsible in part for the proper uptake of calcium into the bones and thus wards off illnesses such as MBD (metabolic bone disease). They can, when exposed to a UVB index of the correct magnitude, produce their own vitamin D3 in a fully natural way without the need for any dietary vitamin D3. Bit what's so bad with just supplying it via the mouth? Well, as previously stated, we do not know how much they actually need and so it is easy to both underside and overdose; if produced in the skin, then the animal can 'stop' the D3 cycle until it requires more or simply go into an area of greater shade.
I would recommend, therefore, not using any supplement with D3 in it and instead using appropriate UVB lighting. I won't go too in-depth about selecting this, but basically, you want to fit a bulb that runs approximately half of the length of the enclosure, situating this at the same end as the heat source so that the photogradient and thermogradient align as in nature (the sun provides heat and light simultaneously, as it were). If you're using an enclosure with a mesh top, get a ~5% output UVB bulb (some brands do 6%, others 7%) and house this in a reflector, above the mesh. If you're using a wooden enclosure then you will only need a ~2% output UVB bulb, but ensure that this is used in a reflector and placed right at the top of it.
NEVER, and I repeat, NEVER put any light source to the side of a reptile, as here you will be faced with eye issues.
Good UVB bulb brands to look out for are Arcadia (the best, as far as I'm concerned!) and ZooMed (which I've never used but are critically acclaimed). You will want to replace the bulb every six-nine months in most cases, although Arcadia bulbs are guaranteed for a whole year.
Bear in mind that you may require a different output UVB bulb if your enclosure is not of a standard size (about 60cm long, 30-60 cm high and 30-60cm deep).
As well as this, using plain old calcium is also somewhat outdated. We now recognise the importance of magnesium in the calcium cycle and so companies are beginning to re-think their old plain calcium powders to incorporate roughly one tenth magnesium. Definitely have a look out for such a product.
On the whole, whilst I won't explain fully why here, I would personally always stick to the supplements produced by Arcadia: EarthPro-A and EarthPro-CaMg. If you use these, then it's as simple as feeding three feeds dusted with A and one feed dusted with CaMg, never not-dusting a feed and also leaving a bowl of CaMg accessible at all times. These supplements do rely on having a proper UVB lighting system installed which, again, I cannot recommend enough. They also have the benefit of having zero synthetic vitamins in them; there's no risk of overdose! Definitely, in my opinion and probably the opinions of anyone else who's used these products, the way forward.
I will note that this is not the only way to 'do it'. Elizabeth's method has been shown, through experience, to work; she's very knowledgeable on the subject and I would not doubt her in what she says. However, the method about which I speak has been proven not only through trial and error but scientific research and that, for me, has to come out on top
