1000ml -> [membrane 1] -> 800ml waste -> [membrane 2]->600 waste
_____________________-> 200ml good ________________->200 good
(ignore the underscore, that is just there to position the text in the right place)
so membrane 1 is operating at 1:4 and membrane 2 is operating at 1:3 via the 550ml/min flow restrictor which is what typically comes with a 75GPD RODI systems. The minimum flow ratio for the membrane is 1:3, that is why 75RODI systems come with 550ML restrictors instead of 800ML restrictors.
so in the above illustration, you get 400ml good water to 600ml waste water, hence I said in my earlier post the ratio is 2:3.
The series configuration is not the same as simply having one long 75gpd membrane as you described.
So the original restrictor does not need to be changed.