how do i add and subtract from a field so that it can be tracked?

how do i add and subtract from a field so that it can be tracked?

example:

Lot code: 11ZN-STE

beginning volume - 1000 gallons

action - barrel racking

ending volume - 900 gallons

wine loss - 100 gallons

Then, the next time i open this field, i would like the beginning volume to be the last ending volume or 900 gallons.

Any suggestions?