Just attack rolls and save DCs. Maaaaaaaaaybe damage rolls. And they'd probably be a little different, since ability scores usually go from +3 to +5. My initial idea was to increase on the odd levels so it's smooth, but that's +10 growth when the baseline grows by +8; if you assumed someone only starting with a 14, they'd grow by up to +9.Just to elaborate: going your way, you could define all d20 rolls by d20 + PB when you're not proficient, d20 + PB + PB when you're proficient, and d20 + PB + PB + PB when you're an expert. That's an abstraction too far for me.