I'm playing through SOA and TOB with a thief, and I've been wondering, what are the points after which further investment into certain thief skills does not help?
I realise that there are traps and lock which add penalties to your skill, so what is the highest penalty? What is the worst penalty for trying to stealth under a light? How high a DI do you need to see through any spells? And at what point does the fail rate for pickpocket no longer decrease? (AFAIK, there are no penalties for Set Traps, are there?)
And just how does the game decide if you are going to succeed? Does it roll, or does it just compare? Any maths would be appreciated.
As would a way to determine these values myself if necessary. I presume that there is some editor at TeamBG which I can examine in-game trap/lock parameters, et al, but Some guidance as to what, exactly would be welcome.

<font color=cadetblue>150-200 is more than enough for Find/remove traps and Pick locks. I believe that there is a 1% chance of failing regardless of your thieving skill. 100 should also be enough for detect illusions and set traps. FOr the stealth skills, the higher you go, the better your skill is so try maxing them out if you backstab a lot.</font>

Are you sure it's just 1%? I thought it was 5% chance of failure no matter what.

I always get annoyed with my characters that have very high pick pockets %ages that seem to fail stealing from a store more often than seems reasonable. This post makes me think that there is more to it than just the straight % listed -- like a hidden penalty. Is that what's up?

I look forward to seeing the answers to Qmanol's and my question.

<span style="color: lightblue">They're all stored as one-byte values, meaning the maximum they can store is 11111111₂, which is 255₁₀. Meaning, it can store any value from -255 to 255. Anything outside that range will either wrap around, read as the extremity, or cause an error depending on what the game was coded in.
There are many many penalities throughout the game, maximum possible penalty should be 255, I would think - anything further than that runs the risk of the penalty suddenly becoming a bonus.
Chance of failure is only in play when rolling to hit and to save. With thieving skills, your chance of failure is 100 - your effective score. Don't make the mistake, however, that then putting 100% in everything means you can't fail - it is based on your effective score. Those penalties change your effective score, so if you have 100 in Hide in Shadows, you put armour on that gives a 40% Penalty, you only have a 60% chance of a succesful hide.
On the pickpocket issue - all shopkeepers impose a penalty.

also IIRC if you download IEEP, and go into the storemaker you can upload the stores and see what the pickpocket needs to be