We use flow meters for many applications, mostly cold sap. These type of sensors work best with a steady flow. They are somewhat less accurate with start/stop type flows. The paddle-wheel or propeller has some initial resistance to flow starting up, and also is prone to continue to spin a bit when the sap stops, so using them in a float-regulated application may result in some amount of error, however it probably isn't enough of a big deal for what you're doing. One thing I would suggest however is to lengthen the straight pipe sections both before and after the meter. Typically you want a straight section (no bends or restrictions) for at least 10X the diameter of the pipe on the incoming and outgoing sections in order to get nice laminar flow. Eddies created by bends or other obstructions will cause systematic errors in the measurement. These types of sensors also typically assume a full-pipe of liquid, which you may or may not have in your setup (hard to say without looking at your float -- put some clear PVC inline before and after the meter to tell).

Only other comment is that you should flush it out occasionally with water (and don't let it freeze). The sugar in the sap could cause the impeller to become sticky and even stop, which could reduce your sap flow going into the pans (not a good thing). Put a tee and valves before (but at least 10X the distance in both cases) so you can isolate the meter and run water through it, or better yet, put union fittings on both sides (again, respecting the 10X distance rule) and take it out and run water through it (gently, don't overspin the impeller).