Make a rule: no change is paid unless you approved it in writing before the work, unless your contract allows another way.
How it flows
Keep a running total of approved and pending changes beside the contract sum.
Review provisional sums and allowances monthly, because they are estimates.
When the total passes an agreed share of the contract, stop and review the scope.
Keep the final cost near the contract price by approving every change in advance, tracking the running total and checking each month's forecast against the budget.
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