Build a parametric assembly

Give the assembly the few named parameters it really turns on. Each is one of three kinds: an input the estimator enters when they apply it, with a sensible default; a constant that does not move, such as a rebar ratio; or a calculated value derived from the others, such as area from length times height.

Why: The parameters are the questions the assembly asks whoever uses it, so keep them to the ones that genuinely change between jobs. Every extra input is another figure somebody has to be right about at half past five on the day the tender goes in.

Set a component's quantity from a formula over the parameters instead of a fixed number, the reinforcement as wall area times the rebar ratio, the formwork as area times a double-sided factor. Any line whose quantity is computed carries an fx marker on it.

Why: A fixed recipe makes you re-derive the steel by hand every time the wall moves. Written as a formula the relationship is stated once, so the ratio you agreed with the engineer is the ratio in every priced line, and the fx marker stops anyone reading a derived quantity as one that was measured.

The editor checks the parameter graph as you work and flags problems inline against the parameter or component they belong to: a name referenced but never defined, two parameters sharing a name, a formula that will not parse, a cycle where two calculated values depend on each other. Then run the expansion preview at real values and read the quantity each line moves from and to, plus the rolled-up rate the server will write.

Why: A cycle or a stale reference does not announce itself in a spreadsheet, it just returns a number that looks plausible. Previewing at values you actually expect is what catches a formula that is right in principle and out by a factor of ten, and because the preview runs on the same decimal arithmetic as the live expansion, the rate you read here is the rate that lands, exact to the cent.

Place the assembly on a bill position and enter the one figure it asks for, the wall area for that element. The components expand at those values and the composite rate follows, with nobody retyping a quantity.

Why: This is where the parametric version earns its keep over a fixed recipe. When the wall grows at the next design issue you change one input rather than eleven component quantities, and the two or three that would quietly have been forgotten move with the rest.