I think it tied to a persentage (I could be wrong)
a Thief with 110 in hide in shadows and 115 Move silently.
One a thick stone floor with partial moonlight equlals best conditions the thief cannot fail his hide in Shadows and he can walk quietly without being heard.
A after noon in the forest with twigs everywhere Imposed a penalty of 30 on move silently and the shadows made by the trees impose a 20 penalty to the hide in shadows. so the thief's abilty is more 85 move silently and 90 hide in shadows.