Is the -res counted from the res you actually have on the gear or from the 75 res on the char screen?

question is for PVP

Always from your uncapped res. So overstacked res work in your favor. Hope that helps.

Have oncreen res of 75 plus let's say 1000 more fire res still reduces the same damage. So only overstack to counter -res like cold mastery and conviction for example

The result u watch on ur stats is end resistance. Included is any active conv aura or lower res.
What you do not see is the negative effects of CM/Facets/Gear etc... And the aura has to be active, meaning you have to be outside town and within range while aura is up.

So without clicking into the advanced stat tab you will see result of what it is at that excact moment. Then you can check ur total resistance in the advanced stat tab. And from them you must subtract the facets/cm etc...