If 80% of cities just have Your religion (no matter if there others) - you win
I'm pretty sure this isn't true. Try it: create a game in worldbuilder with 4 cities (all size 2). Add religion A to all 4 cities, and then religion B to city 1, religion C to city 2, religion D to city 3 and religion E to city 4.
Then start the game and go into the religions screen.
Under your theory, religion A should be 100% influence, and religions B-E should be 25% each.
I don't think thats right. I think that religious influence across all religions sums to at most 100% (it can be less than 100% if some cities don't have any religion).
Under my theory, religion A will be 50%, and religions B-E will be 12.5% each.
I've seen alot of games which were ended too early because crazy FoL spread.
This could happen because there are only 3 early game religions, and they need not be founded depending on the player mix, and religions can be spread *fast*. So if 1 player founds FoL, and there isn't Lanun or any dwarves, then often neither OO or RoK will get founded, so FoL can rapidly spread for an easy win.
Thats not really a risk in Dune; we have 5 religions pretty early, and only 1 spreads fast (and it is easily un-spread).