No construction contract may permit the withholding of retainage from any payment in excess of the amounts permitted in this Section. A construction contract may provide for the withholding of retainage of up to 10% of any payment made prior to the completion of 50% of the contract. When a contract is 50% complete, retainage withheld shall be reduced so that no more than 5% is held. After the contract is 50% complete, no more than 5% of the amount of any subsequent payments made under the contract may be held as retainage.