Is there a linear relationship for diluting RA with RO water?
for instance if my RA was 100 and I wanted to get it to 80 could I do that by making sure 1/5 of my water was RO?
Thanks in advance for your advice.
Almost. Unfortunately, the quality of RO water varies with the quality of the raw water and the condition of the RO system. In many cases, RO water does still have some alkalinity and that means that it would take a little more than a linear assumption to reach your desired diluted alkalinity. In addition, you mention RA. That is a combination of Ca, Mg, and alkalinity and its possible that the linear assumption is again off. You are best finding out the water quality of the RO water being used and plug that into a calculator to find out how much dilution you actually need to achieve your desired water quality.
Another thing that could be done is to use distilled water. Then the linear assumption would be valid and that dilution ratio would equate to the resulting diluted water quality as proposed in your post.