He may be in a situation where you need to chase the numbers - maybe shoot only when he is over 200 mg/dL on a human glucometer.
And if you aren't already doing so, feed in mini-meals (a timed feeder like the Pet Safe 5may help with this) to reduce any food spikes and reduce the workload at any one time on his pancreas.
Or, maybe try Mel's trick to shoot 1 drop - push the plunger in as far as you can, relax your finger on the syringe, and whatever it draws up, if it is less than the 0.25 you've been measuring, will be a lower dose and maybe be a bit safer.