A TDS/EC meter is great for determining when to do water changes, but not suitable for accurately measuring water hardness GH & KH. TDS also picks up nitrates or any other impurities in the water.
I don't mix RO as my problem is the opposite, my water is too soft and I add GH booster.
So I just measure the GH/KH of my tap water and adjust the water based on that in a 32 gal Rubbermaid bucket on rolls and then pump that into the tank.
I measure GH every time, because my water supplier has multiple reservoirs with different hardness they publish the range 0-5 dGH in their water report.
Also make sure your R/O doesn't add salts back into the water.
Last adding 1dGH back in using equilibrium or GH booster in isn't a bad idea, as it ensures you have both magnesium and calcium for your plants.