Enemies can't have less than -100 CR, Cold mastery can't affect anything that is Cold Immune (so 100+ CR) that means that anything over -200 is a waste, since even a 99CR monster will go to -100CR.
Now for practical, -150 is the minimum you want to have, since a lot of monster have 50CR, going up to -175 is also a good option if you farm mephisto that have 75CR and you will still finde a good amount of monster with 75CR thanks to cold enchanted mod.
Going over -175 is less and less useful, the amount of monster with more than 75 but less than 100 are more rare, in any case never go over -200.
This post was edited by Titus1986 on Nov 3 2021 07:33am