First, fix the unit: referring domains, not backlinks.
Backlink counts mislead because one enthusiastic website can generate hundreds of them. Five hundred links from a single blog are one editorial decision repeated, and Google appears to score them broadly that way. Referring domains, the count of unique sites linking to you, is the unit every serious calculation uses.
The second fix is level. For a specific keyword, what matters most is the link profile of the ranking URL itself, supported by the overall strength of its domain. A weak page on a strong domain can rank, and so can a strong page on a modest domain. The gap method measures the page level first, because that's where the fight for a single query happens.