How can a buyer or analyst build a better market estimate?

Start with a defensible denominator and show the exclusions. The Economic Census data-access page provides an official route to industry data, but a broad plumbing/heating/air-conditioning classification does not by itself isolate commercial installation firms of the desired size.

1. Define firm versus establishment, geography, employee range and commercial-installation share. Avoid counting several branches of the same buyer as independent purchasing accounts.
2. Obtain the relevant official industry and size tables, documenting their reference year and suppression. Do not infer a missing commercial/residential split from the industry label.
3. Validate the share with enough eligible quoting workload through a transparent contractor sample. Record exclusions and response bias.
4. Define spend consistently: annual recurring subscriptions, annualised services, or first-year project spend. Separate the three before any multiplication.
5. Apply a measured adoption or willingness-to-pay assumption and report a range. Publish sensitivity to each input instead of one precise-looking headline.
6. Compare the result with disclosed vendor revenue only as a plausibility check; do not allocate a vendor's entire company revenue to this category.

No complete set of those inputs was established in this pass. The responsible output is an explicit gap, not a newly invented total addressable market.