What's your tap water like? GH, KH, nitrates, phosphate etc?
The mixing ratio depends obviously on your tap water. RO has zero hardness so use basic maths. Personally I use a 50:50 mix. My tap water is hard (GH 12, KH 7) so diluting it by 50% brings the hardness down to levels that both my plants and fish prefer i.e. KH 3.5, GH 6.
My tap PO4 levels are also 5+ppm which is a little high even for a heavily planted tank.
I'm still not sure if most plants prefer harder or softer water, obviously to a certain degree it depends on your plants but I know most will grow in hard water as long as there's enough CO2 and other nutrients. Soft water does make it "easier" for the plants to utilise some micronutrients I believe, hard water can oxidise these rapidly.
As a general rule plants that come from soft water habitats can do well in all types of water but hard water originating plants i.e. Vallis, Egeria, Baby Tears and some Crypts (balansae I think) need hard water and will soon die in very soft water.
Your fish selection will prefer softer water but if they've been tank bred and acclimitised to harder water then hardness isn't really an issue unless you want to try breeding.
Personally I'd avoid pure RO in the planted tank. You can buy re-mineralising powders etc. but in my experience they fail to add enough plant nutrients, I've tried Kent and Tropic Marin products with little success. Better to use the stuff already present in tap, cheaper and easier too. You could use pure RO and dose with all the necessary dry ferts including MgSO4 and Ca based to get the hardness and buffering capacity up but this is for the more experienced planted hobbyist I would suggest.
Hope this helps.
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