Draining water ratio: 1:3:

The ratio of product to waste is a production rate to drain rate. If you put a pressure tank and measure how much waste water to fill the pressure tank, then the calculation is different. This ratio will be changing as it fills up the tank, since the drain rate is fixed at 500mL/min. and the production is decreasing as the tank fills up. So when the tank is empty, the ratio is 1:3, then as the tank fills up, it will be 1:4, 1:5 then it goes higher
until the tank is full.

Why did this happen? The pressure from the tank is pushing against the water. when the tank is empty, the tank pressure is 7 psi, as the tank fills up it goes higher so when the tank is almost full it can reach about 35 to 45 psi, so If you have 55 psi going into the RO, the net pressure across the membrane is 55-40 = 15 psi, that's why the system makes water slowly at the end.

If you wish to have less waste, you can purchase a smaller flow restrictor, so it limit the amount of water
going to the drain. However, there is a trade off. If you have less drain water, then the stage #4 membrane filter needs to be replaced more often. Also the TDS reading will be about 5 to 10 ppm higher after the 4th stage membrane. So you need to refill the DI resin more often.

The production rate of a RO system depends on your input pressure and water temperature, so when we say the ratio is 1:3 this is only approximation, it is not fixed. It depends on your water supply.

If you want the membrane to last longer, then have more drain. If saving water is more important, then use smaller flow restrictor and less drain. If you want both, then you need to add a booster pump or a permeate pump to the RO system.