of course if you are at the minimum and see the runway or any of its references, you can land.
However I've never used or seen using the calculation you're talking about.
I'll go check the Jeppesen Introduction, I think there are all the answers to the questions you asked.
Regarding the ILS CAT I minimum, well, they are DH=200ft RVR=/+ 550mt or VIS=/+ 800mt and there's not doubt about it, it's in the introduction as well.
This means that, if you're copying a weather report or reading a weather bulletin reporting only a VIS value, that will be your reference.The same will be if only RVR is reported (pretty unusual...)