If you wish to use RO water then I'd start with a ratio of 50/50.
Reason I said "IF" is because I don't think you may need RO. A TDS of around 140 is fine for most discus, many even breed in this hardness of water. Only reason to be using RO is if the source water has something in it that cannot be removed with ageing or water conditioner such as very high nitrates or phosphates. If that is the case then using aged tap to re mineralise is counter productive, re-min salts would be better. Hope that makes sense, btw I use only re-mineralised RO water for all my discus (and Altums).
Welcome on board too.