Good question.
And there are so many variables.... Page yields etc. were all over the place and no " number crunching" ended in consistent results.
However, strictly focusing on volume (70 ml) watch the following YouTube video link:
View: https://www.youtube.com/watch?v=ySw66VRnggU
Start around the 8:00 minute mark and carefully watch the refills.
Especially Magenta and PB. You can just barely see the ink going down in the inverted bottles. About half way I think.
Poster moves the bottles off-camera as each refill is completed - so you cannot really always see the upright bottles after each tank is filled.
Bottle to tank refill ratio: Looks to me like about 2 refills per bottle.
Google for other similar "how-to's" and "tutorials". Maybe someone will specifically mention the bottle/tank refill ratio.