Since flow meters used on an RO are scaled in gal/minute, whith a 400 gal/day you at most will get 16.667 gal per hour and .2777 gal/minute. That will be total, now divide that into concentrate and permeate, such a meter that could measure accurately might well cost more than all of the other components together.
I think you should just regulate the flows until they look to be 50/50 or maybe 60/40, with the 40% being concentrate, if that set up will do that.
This does not mean building your own RO is a waste of time, by all means build one, just forget the flow meters.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.