How should progress be measured from one period to the next?

Progress should be measured, not estimated, and the difference is the whole point. Measuring means going line by line against the pricing document, the priced bills, activity schedule or schedule of rates, and assessing what proportion of each item is genuinely complete, from the site's own records rather than a feeling that the job is about sixty per cent there. The number is whatever the lines add up to, which is why it is defensible: an assessor can dispute a round percentage forever but has to engage with a measured line that points at a marked-up drawing. Three disciplines make month-to-month measurement reliable. First, record the measure as you take it, noting what was measured, where, and which record supported it, so the figure carries its workings rather than being a bare total re-justified each period. Second, get each period's figure agreed and keep it agreed: when a line is accepted, note it and on what basis, so next period starts from that fact instead of reopening the debate. Third, reconcile the cumulative position each period against what was last agreed, and where a line moves backwards, say so and explain why, because an account whose consecutive periods cannot both be true invites doubt about all of them. Done this way the running total becomes a proven story that reconciles month on month, which makes the final account a formality; done as a monthly guess, the figure is soft, gets cut, and is argued all over again every period.

Take a drylining subcontractor on a fourteen-period job whose progress is assessed each period as a single percentage per area, rolled forward and nudged up. The certifier, unable to see the basis, trims every application, and because nothing is recorded the same lines are re-argued month after month. The change is procedural: each period the subcontractor measures the boarded and jointed areas against the priced schedule, marks up a plan to show what is complete, and writes the measure beside each line. The certifier now checks rather than discounts, most lines are agreed at the measured figure, and an area agreed in period four is not reopened in period five because the record shows it was settled. By the final account there is nothing left to argue: the cumulative position reconciled every period, so the closing number is the sum of fourteen agreements rather than the opening bid in a negotiation.