I don't feel that I can make a particularly useful contribution to this thread, because my supplementation is usually played by ear.
It might sound stupid, but I have found that I have developed a good feel for when to supplement (with which supplements) and when to skip supplementing.
Occasionally, if I have a breeding female that is producing well, I might give them a weak dose of calcium lactate solution mid-season, just as some sort of a top-up. I have only done this once or twice, and have not noticed any detrimental effects of doing so.
I also always give calcium (CaCO3) dishes to my breeding females, as I have seen phantasticus, sikorae and finiavana self-dosing. I think my pietschmanni have been self-dosing as well.
I think that one of the most fundamentally important things to remember is that supplementation and proper gutloading of feeder insects should not be treated as being mutually exclusive, and in order to provide adequate nutrition for the animals, feeder insects (and variety of), gutloading, and dusting of the livefood all need to be properly accounted for.
I think that the fact that this results in multiple layers of complexities and variables, is the main reason that mistakes of malnutrition occur.
Perhaps because of this multi-layered system, some of the deaths we see - that we put down to improper nutrition - could be caused by bioaccumulation/biomagnification of contaminants or toxins present in the gutloading food or the livefood itself. But I haven’t thought this through, it has only just occurred to me as a possibility, so thought I would at least share my thoughts.
Anyway, as I say, I generally go with my gut as to how to provide the appropriate nutrition for my geckos, and thus far I haven't had any problems. The only deaths I have had recently are either animals from one particular disreputable breeder, or the odd hatchling dying from (presumably) normal infant mortality.
I have only ever had one female gecko (u. finiavana) have a calcium crash, but she was extraordinarily productive that particular season (about 10 clutches, if I remember correctly), and after a couple of days of calcium lactate treatment, twice daily, she had stabilised.
After an extended winter cooling period, and after receiving infertile eggs from her, checking her calcium sacs, and when absolutely convinced she was out of the woods, she bred again and had another very productive season, albeit less so than the previous. She has been in top health ever since.
EDIT: Just thought I would add that I have bred and used baby woodlice for hatchlings, and have colonies of small snails in most of my vivaria. I have no idea what species the snail are, but I have seen partially digested shells in my finiavana, phantasticus and sikorae vivaria, so I assume they have been eating them.