New record field plus previous record field

[Matt.p]

Hi Newbie Here. Right first things first im a novice at filemaker so you will have to bare with me.

Right i have 3 fields

Actual strokes used,Amount of chemical,Strokes remaining.

Now what i need to do is take the amount of chemical minus it off of the actual strokes used and then add the previous records strokes remaining, but i have no idea how to put this in filemaker terms

Help