I'm not quite sure what you are asking for, but assuming you mean you want the skill to have, for instance, a greater effect everytime the level is a multiple of 5 (5, 10, 15, 20, etc...), then you could just use the modulus function. So, set a variable to the actor's level. Then mod it by 5. You would then need only one conditional branch which checks if that variable is equal to 0.
Just so you know, what modulus does is essentially give you the remainder of the variable when it is divided by 5.