In DKS starting from D2:ED, Ranger Stealth can not be upgraded past level 5. Starting a new FoV character, it is available up to level 12 in the advanced configuration, and if you max it the cap is 15 (if you put 6 points in, the cap will be 10 once you get into FoV).
Sight Range reduction % : 14, 17, 20, 23, 26, 29, 32, 35, 38, 41, 44, 47, 50 (req level 36), ?, ?
At a wild guess, I'd say level 14 and 15 are probably 53 and 56%.