Clever ! Also ,extrapolating on from that could you not do away with the upper "bottle stop" valve , keep the lower one and have a cylinder connected to a hose feed so that you could fill continuously while the bottom is pushed up ? That wouldnt serve the autofill process so well as having the measured dose that you suggest but might be useful to build alongside .
Come to think of it , dosing guns for livestock have a measured dose cylinder that you wind back to get the amount needed by altering the cylinder length . The OP is a farmer so he will know what Im on about . That sort of system using some plastic pipe as the measuring cylinder would give you the flexibility on an autofill set up