Take 500ml tap water, add 1.26g Potassium Bicarbonate.
Take 985ml tap water and add 15ml of the above concentrate.
Alternatively, add 1g (1.06g) of Sodium Bicarbonate to 500ml instead.
If you use pure distilled water the calculations are a little different, I’ve taken into account the fact you’re making a concentrate with water that already has 19.6mg/l alkalinity.
Best keep the concentrate refrigerated in a sealed container and use up as quickly as possible, and just to be safe boil the water before use - e.g don’t make cold brew with it or just drink it.
You’re getting an additional 8.70mg/l Sodium using Sodium Bicarb over Potassium. Not really sure that will make a difference but you never know….