Its a question that never goes away; here's my take. The waste water is basically concentrated water; the membrane rejects the majority of dissolved solids & this is usually sent to drain. The purity & production rate depends on the quality of the water to start with (TDS reading), the pressure the water is boosted to & available flow rate. By decreasing the rejection rate, you produce more RO quality water, but increase the TDS loading of dissolved solids on to he membrane surfaces/pores. Therefore, if you want to produce as much RO water, with as little waste water as possible, your membrane life expectancy will shorten. You have to make a commercial decision; if I was on a water meter, I'd probably hammer the cheapest membranes available & build in their replacement cost against the cost of water down the drain. If not on a water meter, I'd increase the number of membranes I'm using & run them gently, though mindful if you run at reduced pressures, you may well use more demin resin, as the quality goes down with pressure on the membranes.