The problem with capstones was that a 20 fighter did 10% more damage than a level 19 fighter. A level 20 rogue did 4d6 more damage than a level 19 rogue (that is 7 rogue levels btw).
You can't justify that kind of power increase by one level. That is why I'm saying that you make everything additive instead of geometric.
For example, the Fighter Alacrity Capstone should have 2.5% per tier from level 17 to 20. This makes a Level 20 fighter better than a level 19 fighter but not prohibitively so.