[umbros]

Forgive me for being dense :)

So the Thief with Stealth-18 (w/o Backstabber) would roll against:

13-unencumbered in shadowy conditions
-1 to -4 for encumbrance
-5 if ambushed
-5 in well lit conditions
+5 if ambushing

worst case: well lit, extra heavy encumbrance, and ambushed; Stealth-(-1)(impossible)
best case: unencumbered, in the shadows, and ambushing Stealth-18

The same Thief with Backstabber:
18-unencumbered in shadowy conditions
worst case: well lit, extra heavy encumbrance, and ambushed
(BS 1) 4- Stealth 18-5-4-5= 4
(BS 2) 6- Stealth 18-5-4-5+2= 6
(BS 3) 8- Stealth 18-5-4-5+4= 8
best case: unencumbered, in the shadows, and ambushing; Stealth-18

Does this look right?