Since I'm not very clear about the specific proportional relationship you said, I didn't create the Charge Date column during the test. But in the formula below, I use $3061 to represent the software price after 9.30. I believe your focus must be on the previous loop, you just need to replace $3061 in the formula with your ratio.
Here is the formula I used and the test result:
IF(AND([Installation Date]="Before Sep 30,2020", [request type]="New"), [Lic Qty]*750, IF([Installation Date]="After Sep 30,2020",[Lic Qty]*3061,[Lic Qty]*430))
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