@Metalheadfred: This spreadsheet may be just what you’re looking for.
To estimate how many ppm a plant ate (Appetite):
Appetite = Previous runoff ppm + Latest feed ppm - Latest runoff ppm
So, for example, if a plant’s runoff measured 400 ppm previously, the latest feeding added 600, and the runoff from that feeding measured 300, the plant absorbed (400 + 600 - 300 =) 700 ppm. Divide by the number of days to estimate the plant’s daily uptake.