If I vote for a pool with 100,000 votes and a fee of 15% I get anyway more than voting for a pool with 80,000 votes and 0% fee. Is that correct?
No.
Let's say you have 10,000 votes of your own and to round and make it easy........
If you vote for a pool with a now total of 100,000 votes with a fee of 15% then you get (about) 100 sem per day / 10% of votes - 15% fee = 8.5 sem
(Basically a pool gets about 100 sem per day and you would be 10% of them = 10 sem, but pool charges 15%, so you get about 8.5 of those.)
Let's say you have 10,000 votes of your own and to round and make it easy........
If you vote for a pool with a now total of 80,000 votes with a fee of 0% then you get (about) 100 sem per day / 12.5% of votes - 0% fee = 12.5 sem
I think you were trying to figure out something more like....
At what point is it better to take your 10,000 votes to a pool charging a lower fee than a pool charging a high fee.
Most people don't think about it and simply vote for pools with a low fee.
Not a simple answer since there is going to be a curve in that you want the lowest fee while supporting the highest percentage of votes. (0% fee / 100% votes)
So, it's best to own your own validator if you have enough votes while charging yourself a 0% fee.
With (for the example) 10,000 votes, it best to find a low fee (still relatively the most important - currently), while getting into a pool which barely has enough votes to be a validator (like within the top 100 (easy), but better within the first 56 (like being in position 55).
People voting for pool in position 1 are making less than those in position 55 since they have to share the block rewards with far more people even if the pool in 55 charges a higher fee than that of position 1.