Not clear to me. Hold 10% until 50% and then hold 5% on each of the remaining payments. At end of the project retainage held is 10% of 50% plus 5% of 50%? Or, hold 10% of first 50% and then hold no more so at end of project total retainage is 5%?
There really is no interpretation here. The language you use means to hold 10% of the first 1/2 of all opayments, then 5% of the second half. On the other hand, typical project retainage is 15% overall. What you describe would amount to 7 1/2%. Is that what the parties had in mind?