Yeah, 90% of F7's worldwide gross is about $1.44b, so I tried to account for that with a $1.3-$1.4b estimate. Maybe I need to shave off a bit more to account for it, I don't know.
I think it would be simpler to just set aside the Chinese gross. Fate of the Furious is going end up with a similar final USD gross to Furious 7 in China. Figure $400M for simplicity's sake. I'd give that a +/- 10% error margin, but close enough.
Last I checked, the Chinese weekend total for F8 ended up being around $193M. That means that the rest of the world was ~$340M during that opening weekend.
Furious 7 opened to $397M without China and Russia in its first weekend like you said. If you throw Russia back in to make it comparable to F8, F7's opening was a bit over $410M.
340/410 = .829
So Fate of the Furious opened to around 83% of Furious 7's worldwide opening, excluding China.
Furious 7 made around $1130M outside of China. If you multiplied that the 83% above, you get a bit less than $940M for F8 with the same legs that Furious 7 had. Throw in the $400M from China, and you end up with $1340M.
Fate of the Furious isn't actually all that likely to match Furious 7's legs though. To start with, followups to breakout movie phenomenons are pretty much universally more frontloaded. I can't think of any examples where that isn't true. Furious 7 also had almost an entire month to itself, while Guardians 2 starts launching in overseas territories next Wednesday.
Based on that, I'd probably target $1.25B (+/- 5%)