Most TAM slides are an analyst report multiplied by optimism. A firmographic count is different: it is a list of named companies you could contact tomorrow, which makes it much harder to argue with and much more useful to plan against.
Start from who can actually buy
Write down the two attributes that decide whether a company can buy what you sell. For most B2B products that is headcount and one of industry or technology. Everything else is a nice-to-have that will shrink your count without improving its accuracy.
Count, do not estimate
Build the filter, read the result count, export the list. You now have a number with a spreadsheet behind it. Anyone who disputes the figure can open the file and point at the rows they think do not belong, which is a far better conversation than arguing about a percentage.
A TAM you can open in a spreadsheet is a TAM you can defend in a board meeting.
Segment before you slice
Break the count into serviceable segments by region, size band and industry. That turns one large number into a handful of small ones, each of which maps to a team, a quarter and a message.
Refresh it quarterly
Save the search URL and re-run it every quarter. The delta is your market growth, measured in named companies rather than assumptions, and the new rows are a ready-made prospecting list.