Excel Function to track % of retainage
 

I am trying to figure out a formula that will automatically figure retainage
for pay applications. We keep 10% retainage until we reach 50% of the total
construction cost. After that they are totally retained. My spreadsheet is
set of to retain 10% already. How can I have it check the total contracted
amount and quit removing 10% once retainage has reached 50% of the total
contract amount?
I am using excel 2003.
We usually get billed monthly for six months.

Thanks! Trying to finish this up so I can distribute in the AM.