josh - welcome to PT! And what a brilliant first post! Affordable, reliable, autodosing would be a real boon to this hobby IMO, and I think this is a great contribution!
As you have probably seen, a number of people are working on this problem here. But I'm not sure if they have the accuracy you are talking about. It would seem that your only drawback is the reduction in fluid pumped as the holding tank level drops.
Have you thought about drilling a hole in the holding tank, then using some timed mechanism to top it off daily, so the level stays constant? I know that adds complexity and cost. Just my 2 cents.
But either way, welcome! And thanks for a great