Capillary flow restrictors are pre-trimmed to length to match the gpd capacity of your reverse osmosis membrane. Select the size above that matches your membrane.  These restrictors are designed to provide an approximate 4 to 1 ratio of waste water to purified water assuming the RO membrane is supplied with 77 degree F water at 60 psi.  If you have colder water, or lower pressure, consider selecting a flow restrictor for a lower gpd membrane.