Ask the customer directly during a structured account review, framed as a coverage question about how much of their program you support. Long-tenured customers answer this more often than sales teams expect. For accounts where that is premature, triangulate from operating scale, competitor product your field team observes on site, public expansion and capital spending announcements, and categories the customer once bought from you and stopped. Attach a confidence level to each estimate and aim to be right within plus or minus 15 points.