Basically, Advantage/Disadvantage is huge in the mid range. This is why you kick my ass all the time. You normally need a number in the midrange to hit.

I need a little more time to settle on a character sheet. I did not see any community sheets for 3.5 so I was looking at either the core 3.5 sheet or the pathfinder sheet. Pathfinder sheet: https://app.roll20.net/forum/post/4735667/pf-pathfinder-sheet-thread-6

The True Thief Level… Proficiency Bonus… Sneak Attack… Class Features 1 +2 2d4 Sneak Attack, Expertise (Alchemical), Chameleon Cloak 2 +2 2d4 Cunning Action 3 +2 2d6 Combo I 4 +2 2d6 Ability Score Improvement 5 +3 3d6 Shadow Walk 6 +3 4d6 7 +3 4d8 Weaponstrike 8 +3 4d8 9 +4 6d8 Improved Chameleon Cloak 10 +4 6d8 11 +4 6d10 12 +4 8d10 13 +5 8d10 14 +5 8d10 15 +5 10d10 Join Strike 16 +5 10d10 17 +6 10d10 18 +6 12d12 19 +6 12d12 20 +6 12d12