With your GH being that high, I'd dilute because it can limit the fish you can stock. I used to run straight tap, mine had a GH of 13-15...I couldn't keep certain fish alive. Oto's would only last a few days, neon tetras didn't thrive. I made the switch to RODI and haven't lost an Oto since, other fish seem to do much better in general as well.
I go the remineralization route vs dilution, using CaSO4 and MgSO4 to raise GH to 4-5, and I use nilocG's ReKHarb to raise KH to 3. It allows me complete control and fine tuning. Your well water is free though, and probably fairly consistent. 1:1 mix would be fine, I'd actually go with more RODI though to get that GH closer to 6. Hard enough for all your plants and inverts, not so hard as to limit your stocking.