You've seen
das Durchflußgeber ?
0.396 gph to 39.6 gph for about $60
Problem is that it doesn't quite get low enough for many idlers. If you put that type in parallel then they will see below .396 and get inaccurate. Parallel drip counters might work (how many drips per second can one count reliably?)
re: 555: a $3 pic can replace a whole army of them.
So is there some physics invloved to limit max drips per second before they change volume or otherwise are better described as a stream?