I had a similar situation with a peristaltic pump when the rubber bit that goes around the rotating head is a bit loose / long / not tight enough to keep a good seal.
The way to be sure would be to put a cup under the tubing, open to the air, if that's the way it sits in your sump, and measure like 1/2 a day of cyclic dosing, see if it matches the calculation.
Then you could work up your own 'calibration' for want of a better term, and correct as necessary to make sure you dose the amount you need.