Graded success for roll-under d20

This is a sort of half-baked add-on inspired by Call of Cthulhu 7e's graded difficulty system for roll-under d100 resolution. It assumes 3d6-based ability generation a la BX, and uses the same modifier scale, albeit towards a different end.

Basically, any ability score rated 3-18 has a "crit" range from 1 to (4 + ability modifier):*

Ability Score	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Crit threshold	1	2		3			4				5			6		7

You could also implement a simple skill system:

UNSKILLED:
   Miss: Goes badly
   Success: Works, at a cost.
   Crit: Works
COMPETENT:
   Miss: Doesn't work - can push at risk
   Success: Works
   Crit: Works well
SKILLED:
   Miss: Works at a cost
   Success: Works
   Crit: Works well

So what to do with all this? I dunno there's plenty of good games out there, play one of those instead. You're welcome.