You determine the value for X and announce how damage will be divided as you cast Impact Resonance. Each chosen target must receive at least 1 damage. Once the value of X is determined, it won’t change, even if a source deals a greater amount of damage before Impact Resonance resolves. (Most of the time, the value of X in an effect would be determined only as the spell resolves. However, the rules state that if damage needs to be “divided,” that information is checked as the spell is put on the stack.)