I'm not sure which orifice you're pulling your figures from, but the average legs domestic is 2.3-2.4x for a F&F movie. That would put it at 141M. I can't be arsed to look up what it does legs-wise OS so lets just use that 2.3 as a base line, which puts OS at 282M, combined together for 423M.
Remaining markets with figures dropped equally from F8 to Hobbs as in America (-35%):
China - $255M
France - $20M
Belgium - $4M
Italy - $11M
Korea - $18M
So altogether this pessimistic prediction, ignoring the fact the movie has 0 competition for an entire month, would put Hobbs & Shaw at $731M. So how is $750M unlikely?
And since when did anything under $750M mean a bomb?