New OSR Thieving Mechanic

The whole percentile dice thing is sorta wack.  I mean it's alright, but its really the only time that the dice are used in game by characters in old school play.

I really dig Dyson's thieving skills, but I am thinking of something even easier.

All thieving could be a ability check, specifically a dexterity ability check.  Obviously all characters have dexterity scores, so I need to figure out a way to make thieves slightly better at all things thieving.

Roll under dex and add the following modifier to the roll based on level. Obviously a roll of one is great success! and a twenty is a total epic failure.

Thievery covers the following old schoolisms: open locks, find & remove traps, pick pockets, move silently, climb walls, hide in shadows, hear noise

EDIT:  +Zak Sabbath suggested that this idea could be scaled with the thief's backstab ability, which is a good idea!

Level	Modifier to thievery
1	-1
2	-1
3	-1
4	-2
5	-2
6	-2
7	-3
8	-3
9	-3
10	-4
11+	-4 no change