Creating Model from Data Sample

This proposal employs the What If Analysis > Goal Seek.
1. Find the corresponding Max Percentage value using (cell B18): =INDEX(B3:XFD3,MAX(B1:XFD1)/2)*MAX(B1:XFD1)/MAX(B13:XFD13)
2. Identify the month(s) that should receive that value using (B19:C19): =IF(MOD(MEDIAN(B13:U13),1)=0,MEDIAN(B13:U13),MEDIAN(B13:U13)-0.5)
3. Make a guess as to the ratio by which the Max Percentage should be retained in reduction in preceding and succeeding months (B20)
Note that I originally chose 90% based on 18 of the 20 months not receiving the max percentage and that percentage was changed by Goal Seek to 83.24%.
4. Total percentage sum (B21): =MAX(B16:XFD16)
5. Total revenue (B22): =SUM(B14:U14)
5. The percentage row (15) is populated using: =IF(OR(B13=$B19,B13=$C19),$B18,IF(B13<$B19,C15*$B20,IF(B13>MAX($B19,$C19),A15*$B20)))
6. The percentage sum row (16) is populated using: =SUM($B15:B15)
7. The revenue row (14) is populated using: =$V14*B15
8. In Goal Seek set the Set Cell to B21; the To Value to 1 (or 100%) and the By Changing Cell to B20
Note that Goal Seek took several minutes to come close to 100% (100.01%).
Note that you may choose to make a manual adjustment to such as subtracting $7.40 from cell U14.
Let us know if you have any questions.