Some teams like the Arizona Cardinals still have a number of the $480,000 minimum salary people hitting their top 51. Not so the Patriots, of course, because they pay more for the bottom of the roster players than other teams do.
If you explicitly work it out, assuming the cap hits are correct, the space needed for the Patriots draft picks at this moment is $1,964,677. The shorthand estimate formula comes up with $2,668,148. If you adjusted the short-hand estimate formula to reflect that the Patriots lowest top 51 cap hit right now is $630,000 you get $1,618,148 instead.
Are the differences between these 3 sets of numbers enough to get worked up over? Let the Patriots, NFL, and Miguel worry about the technically correct number. The key point behind the formula is to allow the average fan to realize that the Patriots really only need between $1.6M - $2.7M of cap space for their rookies, not $6M. *That's* a significant difference. Whether it's off by a few hundred thousand isn't important for the average fan.