The simple answer is to 'do what it says in the contract' which means the PM assesses the share amount at Completion (of the whole of the works) and when the final Price for Work Done to Date and final total of the Prices is established.
This means that there is no contractual provision to make an assessment at any other time, although you would want to calculate this anyway for your own financial reporting purposes.
Because of this situation many contracts are amended to allow the PM to make an 'interim' assessment when the PWDD exceeds the total of the Prices, taking account of any likely adjustments to the Prices, such as CE's accepted but not implemented..
This means that there is no contractual provision to make an assessment at any other time, although you would want to calculate this anyway for your own financial reporting purposes.
Because of this situation many contracts are amended to allow the PM to make an 'interim' assessment when the PWDD exceeds the total of the Prices, taking account of any likely adjustments to the Prices, such as CE's accepted but not implemented..