How does the NEC4 Option C Contractor's share work?

The Contractor's share is what makes Option C a target contract rather than a cost reimbursable one, and clause 54 sets out how and when it is calculated. Under clause 54.1 the Project Manager first finds the difference between the total of the Prices, the target as adjusted by implemented compensation events, and the Price for Work Done to Date, the Defined Cost plus Fee actually assessed. That difference is then divided into the share ranges stated in the Contract Data, the share percentage for each range is applied to the part of the difference that falls in that range, and the results are added to give the Contractor's share. Under clause 54.2 the direction follows the sign of the difference: if the Price for Work Done to Date is less than the total of the Prices the Contractor is paid its share of the saving, and if it is greater the Contractor pays its share of the excess. Under clause 54.3 the Project Manager makes a preliminary assessment of the share at Completion using forecasts of the final Price for Work Done to Date and the final total of the Prices, and it is included in the amount due following Completion. Under clause 54.4 the share is assessed finally, using the final figures, and included in the final amount due. The share is not assessed at every interim assessment; until Completion the Contractor is paid its Defined Cost plus Fee in full. What goes wrong: a single percentage is applied to the whole difference instead of range by range; the target is stale because compensation events were never implemented, so the Contractor pays for events it was entitled to; Disallowed Cost is treated as reducing the target rather than the Price for Work Done to Date; and the preliminary assessment at Completion is forgotten, so the whole share lands as one figure in the final assessment.