The % expo depends on the plane. Some planes have tons of rudder and/or elevator authority by design (eg Morris Knife, Goldberg Extreme).

A typical starting point I use on a new plane is:

35 degrees throw and -20 Expo all around on low rate
50 degrees throw and -20 Expo all around on high rate

That said, all my planes have different expo on each surface depending on that surface's authority. So it's just a starting point.

My Global Freestyle has +10 expo on its huge rudder because it lacks authority.