I've been out running around today and also looking at other excel questions.
Let me try to explain the formula I have in Post #2:
=or(AND(L6="",F6<>"",F9<>"",F12<>"",SUM(F6,F9,F12),I6<16,MOD(I6,1)=0),AND(I6>=L6,F6<>"",F9<>"",F12<>"",SUM(F6,F9,F12),I6<16,MOD(I6,1)=0))
Your new question was:
if L6 has a value of at least 1 I don't want someone going back to I6 and changing to a lesser value than the condition value to allow entries into L6. Is it possible to do this?
original rules:
I6 =AND(F6<>"",F9<>"",F12<>"",SUM(F6,F9,F12),I6<16,MOD(I6,1)=0)
For I6: Only format when sum of (F6, F9, F12) is not zero & all 3 are populated, I6 is less than 16, and I6 is a whole number
L6 =AND(I6<>"",I6>9,L6<21,MOD(L6,1)=0)
For L6: Only format when I6 is populated with a whole number gt 9 and L6 lt 21
So, my thought was that your new requirement "really" didn't make any difference of being at Least 1, but only that the value of I6 must be greater than or equal to I6.
So, I just added a second validation rule that is almost like the first for I6, instead of being sure L6 is blank, to also make sure I6 is greater than L6.
I hope that makes sense. And I"m not 100% sure it will catch all your scenarios. Please test it out.