Yes it is simple just use
Bottles to day =
Int ( Amount used today / Bottle size ) //number of whole bottles used//
+ Mod ( Amount used today; Bottle size) //part of bottle used//
and make sure the "Do not replace existing value of field (if any)" box is unchecked. if you use this as a field formula and not in a script.
this needs to be stored separate from the "Amount used today" or you will have a big headache trying to keep it straight.
Thank you very much. My problem is solved.