This is good stuff, but what probability are you modelling, just the T formula and not the whole mechanic? There's the extra check on exit that will cut total success probability a lot.
Thanks. As you said, at the end the exact same check but with ValueTerm = 0 is made. So whatever formula for main check we use, it will still be difficult. Maybe it would be a good idea to remove this last check completely? Pickpocketing is risky even without it. Two checks even with maximum chance of 75% is pretty bad. Moving maximum chance to 90-95% may improve situation, but leaving just one check may be better.
If you check my xls, you will find some estimates on a few sets of skills/stats.
I would describe it like this:
Skill 0, Stat bonus 0. Braindead character, training dummy. Hard to imagine something like this in original game, but it is a start point.
Skill 5, Stat bonus 10. Most clumsy and slow character available from start, something like Chewbacca. Shouldn't be able to pick anything, can be picked easily.
Skill 20, Stat bonus 15. Average starting character. Should be able to pickpocket at least a vegetable, should be able to spot some slow clumsy Chewbacca-thief.
Skill 40, Stat bonus 20. Best thief you can make from start. Has some experience, can pickpocket a merchant, while being able to spot a common thief himself.
Skill 60, Stat bonus 25. Skilled crafty thief. Can pickpocket another thief and spot him easily.
Skill 80, Stat bonus 30. Master-thief. Can steal your beard without you noticing. Extremely difficult to pickpocket by normal means, need some unusually hard ways.
Skill 100, Stat bonus 30. God of all thieves. For him you will pickpocket yourself and then consider him your friend till the end of your life. Need another one like this to pickpocket him.
Distribution of skill seems to be pretty much even across various NPCs. For example, there are plenty of merchants with sneak under 10 and many pawnbrokers with sneak around 40-70. And sneak ~100 seems to be exclusive to some mighty NPCs like Divayth Fyr. So a thief in need will always find himself a target with appropriate stats to pickpocket.