I have not had a good experience recently with a ZooMed calcium product. I have no experience with this exact product. But not ALL supplements are created equal! As many know, we have to house our animals indoors much of the year. In the past we have always used Rep-Cal calcium with D3. Last fall we switched to ZooMed's calcium with D3. It had devastating effects. What we learned was ZooMed's calcium with D3 has 20 times, YES 20 times LESS vitamin D3 than Rep-Cal. Needless to say, I am not pleased with ZooMed and am boycotting their products! It was my mistake for overlooking this minor detail, but I am irritated that they made something so inferior.
You'll get the best value and quality by crushing human multivitamin tablets and offering a bit of it.
Also, there is not a lot of research that supports the idea that vitamin D supplements completely replace the need for UVB, and if you DO offer D supplements, the liquid/oil versions are more potent than the powdered. http://www.puritan.com/vitamin-d-326/liquid-vitamin-d-3-5000-iu-030405
I use absolutely no vitamin supplements whatsoever and only offer calcium as a "side dish", not coated on their food. For the most part my tortoises are outdoors 6 months out of the year and indoors the rest of the time. I've doing so for 25 over 25 years and this is a few examples of what I've raised from hatchlings:
I am not a big fan of "supplemental" vitamins. I believe that a good quality, varied diet along with exposure to natural sunlight (and artificial light when needed) along with adequate hydration grows healthy smooth tortoises. I also have not taken a tortoise to a vet in well over 20 years, haven't had an established tortoise with any respiratory problems, no issues with poorly formed or thin shelled eggs, etc, etc...
GB is right- the goal is to avoid any supplementation by offering a good quality diet.
How this works out is sort of up to you. If they are outside in a decently planted habitat with good graze much of the year, and they get an decent indoor diet with good variety, then you should not need much, as in someone like GB's case.
I supplement roughly a small pinch every couple weeks in the winter because I am fairly limited on what I can get at the local store and my outside time is rarely more than 4 months long lately. I also have to admit that I am having a heck of a time getting stuff to grow that they will graze on.