I have a sheet that i want to stop calculating a field after it has been calculated

Hi there, my sheet is Calculating the BE column =IF(Chr.lvl<BF21,"",Skill1*4) if skill1 changes, it calculates previous cells. if your Intelligence modifier goes up it calcs it as it goes. you can't learn something new from the fourth grade when you have already graduated from high school.
Any other suggestions would be greatly appreciated