kH is not linked in any way with gH. You might be confused about that as you might have come across texts that refer to a total hardness which is not that applicable in artificial conditions such as an aquarium. This confusion is caused by assuming all water hardness comes from CaCO3. That's the link between kH and gH if you want to put it that way. CaCO3 will raise BOTH of them as it contains Calcium and Carbonate. However, this is the only relation between kH and gH. Otherwise they do not influence each other.
Both of them can be raised or lowered independent of each other. For instance, peat will lower kH as it's organic components tend to bind Carbon but it will do nothing to your gH.
My tap water is pretty close to yours and I found a 4:1 ration tap:RO to be good for plants. 50/50 might strip the water of minerals more than necessary. Use your tests to get the water close to what you want.
If the plants are not difficult don't bother with RO water. One thing to keep in mind is that gH tests do not tell the whole story. They just measure how much metal ions you have in the water. gH reading may come from Sodium as well and also does not show the ratio between elements.