Agree the uncontentious ninety per cent first

Most of a final account is not in dispute at all: measured work both sides can see, variations with clear instructions, sums already certified. Get all of that agreed and recorded early, so the argument narrows to the genuine handful of differences. A settled ninety per cent also changes the tone: the rest is a small negotiation between people who have already agreed on most things, not a standoff over everything at once.