That's arbitrary. One or both, enough to make the fund solvent long term. This is a very simple concept. MUCH simpler and more effective than trying to make a sand grain sized dent (at best) by going after federal workers.
Perdonally, id raise ss taxes but thats politicsl suicide so, if not, I'd then start by raising the full benefit age from 67 to 69 and see what that does to the fund.
If it still isn't solvent then decrease the benefits.
However, there is an infinite # of ways with combinations of degrees with which to accomplish this.