Hello omerizm,
To be honest I don't know how you can add minerals to water, and not raise the EC. How much water are you needing? Can you dilute your source water with bottle distilled? You can also create your own water filtration system cheaper than buying a RO system. The cartridges run between $8 and $15 ea, and the cartridge canisters $20 ea (they hold the replaceable cartridges). You can even get inline some inline filters, like for the water line to the ice maker on the refrigerator, or patio misting systems etc. etc.. But I do know of a free
nutrient calculator that might be able to help figure things out.
HydroBuddy v1.40 : The First Free Open Source Hydroponic Nutrient Calculator Program Available Online) | Science in Hydroponics