Calculate 10% up to a specific total amount is reached and then stop

Not quite sure I'm following how it goes from 10% to 5% then to 0%, but sometimes goes from 10% to 0%. This sounds like multiple scenarios. However, if you are just looking for a conditional check... then using an If statement will work.

Something like this [insert your own cell ranges]:

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Most of our contracts we start out holding 10% Retainage of the whole contract amount. There will be times when the contract stipulates that when we have reached 50% completion, so when I have billed out 50% of the total contract amount, we can then bill for 5% of that Retainage and then from that point only continue to hold 5% for the remainder of the project. This one I can manually calculate. However my VP wants me to create the spreadsheet so that on the contracts where we hold 10% up until we have completed 50% of the contract amount and then we no longer hold anything in Retainage, except that initial 10% which is the retainage based off the 50% of contract amount. He wants each line that we have an amount on that ends up totaling the total contract amount to have a formula that stipulates that if we have billed 50% of the contract amount then the line will not calculate anymore retainage on each line. It will only hold the initial 10% of the amount we have billed for on each line. I have attached a snapshot of the current form and also attached the Excel file, so that maybe seeing it will help clarify what I am speaking about. Attachment 629605

After the total from each line item reaches $27,500 total, the retainage then moves to 0% and only $27,500 is held on the project until project is complete

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What does it mean?
It would be great if you fill the table manually