I have been thinking about using a skill system in my next campaign; I was trying to come up with a couple of skills, when I got this idea. The general rule in most OSR D&D-esque games is that parties are surprised on a roll of 1-2/d6, which is essentially a d6 roll against a Target Number (TN) of 3; these numbers are hard coded, but I wanted to play with them a little, which resulted in the following:
Alertness could be a skill which decreases the chances of surprise (by increasing the die size step by step); it could be rolled against 3 by default or higher in case of stealthy creatures (5 for giant spiders lurking on the ceiling, for instance).
Stealth, on the other hand, could be a passive skill, increasing the TN the opponents have to roll against to avoid being surprised; the TN is 3 by default, 4 for skilled, 5 for expert, and 6 for master skill level.
Although noting these numbers for individual monsters seems like extra work, it is actually already included in the rules (counting the chances from 1 and up, though). Any possible downside of this I do not see? How would you try to implement such a rule?