Hi,
Unsure of reason(s) why, about mains voltage and multimeter, self or multimeter...? I see that as a laudable approach, don't get me wrong. Things I prefer not to touch, just in case, the multimeter gets attached on highest/relevant setting before turning on/plugging in whatever's being measured, that way a) I don't have to touch it, or accidentally short something placing the probes which slip or go to the wrong places and inadvertently kill another tester and component..., b) no need to stand on a rubber mat with one hand in the pocket (joke, but the hand method is important to know). Those hook-on tips are really useful, failing those, the terminal strip in the photo could be extended and the wires of interest jumpered to another set of terminals where you screw the tester probes in.
Is this for the SSTC?
Oh yeah, I think your question about the calculations, perhaps to a great degree, can be solved by applying nothing much more complex than delightful Ohm's Law principles to the different sections considering them in steady state, I think that's maybe in part but not entirely what CataM was explaining. And perhaps, but maybe not, SMPS formulas might provide the kind of formulas you are looking to understand. This is one: Power Stage Topology Reference Guide (slyu036), but you have to sign in for it, so I suppose I'm not meant to upload it here... Short one is **broken link removed**