While 'fall-off' can be a sign of trouble, it also can be a sign of nothing. Both Moana and Homecoming had big fall-offs. Indeed a few on these forums were practically gleeful that they seemed destined to fail, but, they both had legs and wound up after a month doing quite well.
Fall-off can be also difficult to discern when a movie has a top-heavy opening weekend. Take an extreme example: if every single person who was going to see a new film in its opening week didn't spread out their viewing through the week but all showed up on Thursday, then it would have a 100% drop off for the rest of the week!
A Star Wars movie is expected to have its more ardent fans see it as soon as possible driving up the opening numbers (for which TLJ broke records behind only TFA), which then makes the rest of the week pale in comparison.
So, the drop-off numbers are not as predictive as they might be.
The best indicator of success, historically, is the CinemaScore, which is an A (from the audience!).
This metric would, judging by TFA's opening weekend mean a final domestic number of about $750-$830 million. TFA's total domestic wound up at $937 million. This would put TLJ at 80-89% of TFA. Which is probably the expectation.
Thursday's B.O. was up 6% from the day before.
Foreign B.O. is at $314 million. That's 5% more of the domestic number. But, it doesn't include yet Monday - Thursday or any of China.