Welp, I've tried running actual numbers based on what info I can find on the F9 FT 1st stage. Grabbed most of my specs from the

F9 Full Thrust and

Merlin 1D Wikipedia pages.

- Dry mass - 22200 kg
- Liftoff mass - 433100 kg
- Prop mass - 410900 kg
- M1D chamber pressure - 97 bar
- M1D Isp - 282 s
- M1D thrust - 845 kN (sl)

Given those M1D numbers, the mass flow rate is roughly 305 kg/s per engine at full throttle.

I got some loose timings from the CRS-12 launch, which can be divided into several phases:

- Liftoff to Max Q (roughly 78 seconds), 9 engines at full throttle
- Max Q to MECO (roughly 67 seconds), 9 engines throttled down to 80%(?)
- Boostback burn (roughly 10 seconds), 1 engine full throttle
- Re-entry burn (roughly 12 seconds), 3 engines full throttle?
- Landing burn(roughly 30 seconds), 1 engine 70% throttle

I'm pulling the throttle amount for Max Q out of thin air. I know they throttle down, but not sure by how much. 80% seems reasonable, but again, that's just pulled out of thin air.

Based on those numbers, I come up with remaning propellant at the end of each phase:

- ~196317 kg
- ~48860 kg
- ~45803 kg
- ~38100 kg
- ~31681 kg

So, definitely more than a couple of hundred kg left in the tanks at the end of the landing burn, so my assumption was pretty wrong. Then again, that assumes the above calculations have any basis in reality.

But it makes sense - given the T/W on the M1D, they need all the ballast they can get, even at 70% throttle.