Take off in metric and buy in imperial

Set the scale from a known dimension on the sheet and measure lengths, areas and volumes in metric, taking each value from the drawing itself rather than from a figure somebody has already converted.

Why: The drawings are metric, the model is metric and the specification is metric. Measuring in one system and converting at every step adds a rounding at every step, and roundings compound worst in the quantities that matter most, which are always the biggest ones.

Keep the measured quantity in its metric unit as the stored value, and treat an imperial reading of it as a view of that number rather than as a replacement for it.

Why: There is only a right answer for one stored number. Storing a converted figure and converting it back is how a quantity acquires a rounding it never had, and nobody catches it, because both versions look entirely reasonable.

Write the nominal product name on the item beside the metric quantity: a 2x4 with its dressed size of 38 by 89 mm stated from the standard, a 4x8 sheet with 1219.2 by 2438.4 mm stated and 1220 by 2440 as the conventional designation. Take each stated size from the standard or the arithmetic that produced it, never from the name.

Why: The quantity is what you measured and the name is what you order, and they are two different facts about the same row. A bill carrying only the metric size has to be translated at the counter by whoever answers the phone, and a bill carrying only the name cannot be checked back against the drawings at all.

Validate the bill before it goes out, looking for the two failures this scope produces: a quantity whose unit no longer matches the way it was measured, and a stated size that has been recomputed from the nominal name instead of taken from the standard.

Why: Both failures produce numbers that look right, which is exactly why a read-through does not catch them. A member sized at 50.8 mm where the standard says 38 mm passes every eye in the room and is wrong by a third, and it is wrong in the direction that costs money on site rather than in the estimate.

Export the bill with the metric quantities intact for the client and the consultant, and produce the material order carrying the nominal names for the supplier, both from the same rows.

Why: Two readers need two different things from one set of rows, and neither should be handed a converted version of the other's. Exports staying metric is also what keeps the bill comparable against the drawings it was measured from, which is the only check anybody can run on it later.