Okay, I rolled a 1 on my Excel (Int) check after a few hundred tries.

Turn on circular references under Options -> Formulas -> Enable Iterations


For BE21:
=IF(BE21<>"",IF(CHR.lvl>=BF21,Skill1*4,""),"")

For BE22 and onwards:
=IF(BE22<>"",IF(CHR.lvl>=BF22,Skill1,""),"")

With the circular reference and iterative calculations turned on, the formula will check if the cell itself is blank, and if it isn't it will update. This will prevent values from changing once they already have skill points assigned for those levels.

I'm giving myself 500 xp.