I love ACKS. But thieves in pretty much all old school D&D-type systems are irritating to me. They really are. Their abilities are so lackluster at low-to-mid levels that players are typically afraid to use them. I’ve been tempted to just give them a high chance of success off-the-bat and then attempt to codify some modifiers for situation or environment for each skill. Then I’d increase that already high chance of success every couple of levels until they had a base 5% chance of failure (before modifiers) by the time they were around name level. Why not? Is a thief that can hide in shadows and move silently reliably right off the bat that much of a powerhouse? They still have a d4 hit die, light armor, and a mediocre attack throw. I don’t mind if they start off with a good shot at getting a x2 backstab in during a combat, or think they can pick a pocket at first level without a 45% chance of alerting their mark. I know it’s anti-old school, but there. I said it. Let the thief be thiefy right off the bat.
Why not give them a flat 75% chance (a throw of 6 or better) at 1st level to succeed at their skills in a favorable environment/situation. Then give them a bonus to succeed if the situation is optimal and a penalty if it is less-than-favorable. A “1” is always a failure and a “20” is always a success. That gets the thief engaged in using their environment or weighing a situation to see how they can make the most out of it. They know it will affect their chances to succeed, which in my opinion is exactly what a thief should be doing - taking it all in to see how to stack the deck in their favor, and making the most out of a situation that they can. Currently a low level thief is reduced to gulping and rolling the dice to see if they get an 18 or better. And climbing walls with some confidence. Otherwise they hide in the back and siphon xp, or no one wants to play one unless you start the game at 5th level. I see a lot of thief henchmen and very few thief PCs, particularly in OSR games with no multiclassing. Ahem. I know there are folks out there that love thieves and love the insane challenge of them and what I’m saying is blasphemy - all I can say is that I read their messages on boards all the time but I don’t think I’ve ever actually seen an enthusiastic, risk-taking, low-level thief in a game unless they were playing a multiclassed thief/something-or-other or were in a 1-shot.
Move silently would be modified by the surface they’re walking on and ambient noise, open locks and find/remove traps by the complexity of the lock/trap and/or the speed at which they attempt to open/find/remove, hear noise modified by obstacles, ambient noise, and the sound itself.
Ultimately, I’d rather hear a PC thief say “Yeah, of course I can open this lock, but it’s going to take some time. Watch my back and look out for trouble.” instead of “I only have a 15% chance to open this lock… where’s the mage and does he have knock?” Let the risky rolls come when the situation demands it and the thief is trying to work their trade in a less-than-optimal situation, like when an ogre with a big club is trying to squish them flat.