Jack from TOTGAD has an attractive house rule whereby he uses the “hear noise” d6 chance as the system for resolving all thief skills. This seems pretty reasonable to me, especially assuming that the alternative is the fixed percentile progression given in OD&D or B/X. All of the percentile skills start off rather low and slowly increase as levels are gained, with arbitrary differences between the various skills and no option to specialize in one skill over another (unlike the point buy systems of, for example, LotFP or Second Edition AD&D).
Looking at the OD&D thief (in Supplement I: Greyhawk), why does open locks start at 15% and move silently start at 20%? Do we really care about this distinction, given that all the skills start out at roughly the same level and increase at approximately the same rate? The one exception is climb walls, which starts out at 87%. But “always use hear noise” with perhaps one special case for climb walls is still far simpler than the official multiple stat percentile system, with functionally similar outcomes.
The schedule of thief hear noise improvement (from Greyhawk, page 11) is as follows. (Thanks to ODD74 for discussion about interpreting hear noise in the OD&D context.)
|Effective d6 bonus||+1||+2||+3||+4||+5|
However, just when I was about to throw in my lot with this method, I realized that it does not allow for the variable degrees of success that I use with the old percentile system. That is, using d6, and without another roll, how do I distinguish between “no progress” and catastrophe? 16% (1 in 6) is too high of a chance for critical failure. In my current approach to thief skills, this is pretty important. Basically, if you fail your thief skill roll, but don’t roll 96 or higher, you don’t obtain your objective, but you can try again. The top 5% is the equivalent of rolling a natural 1 on an attack roll.
According to Greyhawk (page 11), pickpocket or move silently is the most favorable thief skill column, so one could just use that as a general thievery skill similarly to how Jack uses hear noise and preserve the 5% fumble chance.