Quick question about making adversaries
I want to create an adversary that has the Heavy Hitter - Reaction feature. The wording in the CRB (pg 195) says "When this adversary deals damage with a standard attack, you can spend a Fear to gain a +X bonus to the damage roll. I know that X is a number that "differs for each stat block" but how do I know what that number should be if I'm creating it myself? (Is there even an adversary that includes the Heavy Hitter reaction I can use for reference? I couldn't find one).
This also goes for the other Features that include X values. Is there some standard I should be noting, or do I get to decide? And how do I know what a good number is?
Also I know about the Homebrew kit and I also downloaded RightKnighttoFight's guide, I just can't find the answer to this probably stupid question.
Thanks!