Comparing referring domain counts against competitors is the standard way to size a link gap, with two important caveats. First, compare against the sites actually outranking you on your commercial queries, not against the market leader whose profile you will never match. Second, remember that indexes disagree substantially: the same site can show 400 domains in one tool and 900 in another.
Growth rate matters as much as the total. A profile gaining a handful of relevant domains every month reads very differently from one that gained six hundred in a fortnight two years ago and nothing since — the first looks like a business people talk about, the second like a campaign.