I have a table of the experience values required to reach the next level for levels 1-9 and 12-21, plus interpolation for levels 10 and 11 (which I made for FE 1.32, so it might not be entirely accurate anymore - I have not checked). I posted it in one of the other champion experience threads, if you want to look for it. I sum them up and add in the bonuses where required, and then look for the point where the experience gain from the Potential/Knowledge traits is sufficient to give you enough extra levels that you compensate for investing some levels into those traits. It's a comparison of how much unboosted experience equates to how much boosted experience, with an assumption that at the points where you've gained experience-boosting traits you had no residual experience from the level-up.

For example, a champion who takes Potential I at level 3 and doesn't take any other experience boosters will have gained sufficient extra experience to be at level 21 at the time when they would normally have reached level 20.

Edit: the table is in this thread: https://forums.elementalgame.com/443416/