Quoting from measurements is what protects the margin. Wall area, number of coats, preparation hours and the materials each imply: estimated by eye, the same job comes out twenty per cent apart from one week to the next.
Materials disappear into the van. Paint bought for one job and finished on another means neither is costed properly, and by the year end nobody can say which kind of work actually pays.
One rained-off exterior job moves everything. Rescheduling a week of outdoor work in a diary that also holds interiors is the difference between a lost week and a rearranged one.
Photographs before and after are the cheapest insurance in the trade. The state of a wall before it was filled is impossible to argue about afterwards, and it also becomes the gallery that wins the next job.