Here is a link to the "70/30" recipe:https://s3-ap-southeast-2.amazonaws.com ... recipe.pdf
And here's a discussion thread of it on HB:70/30 Water
70-30 uses the same ingredients as Perger's recipe discussed here, but uses far less Epsom salt and therefore has lower hardness and lower sulfate, also slightly lower alkalinity. (It works out to GH:KH of 25:42 mg/l as CaCO3, and [SO4--] of 24 mg/l.) You can take Perger's recipe and simply decrease the Epsom salt to get water that is closer to the "70/30" -- for example if you use half as much Epsom salt you would get a hardness of 47 mg/l as CaCO3 (all magnesium hardness) and a sulfate of 45 mg/l [SO4--].
You could, of course use potassium bicarbonate rather than sodium bicarbonate. To get the exact same alkalinity you would use a bit more, by a factor of 1.2 (because KHCO3 has a molar mass of 100 g/mol, and NaHCO3 has a molar mas of 84 g/mol.) In the Perger recipe, if you use baking soda, you end up with 23 mg/l of [Na++], which is above the sodium recommendation by SCAA for "superior brew" but within their spec for adequate brew
. Some people prefer potassium bicarbonate, for taste and theoretical reasoning -- because of the fact that coffee beans are low in sodium but very high in potassium, you can theoretically use a lot of potassium in the water without expecting much effect on the outcome.
That water spec is interesting in that it recommends 25-50 mg/L of sulfate as being optimal. I've never seen anyone else recommend that the water should include sulfate. It's not clear if they think that going above 50 mg/l is a corrosion issue. La Marzocco's Scott Gugliemino is very clear, however, that chloride is a major corrosion concern, as is "aggressive water" which Scott seems to define as any water low enough in calcium carbonate to have an LSI index lower than 0.1. The Perger water would certainly be aggressive water per that definition. This spec specifies a minimum calcium hardness of 3 gpg (51 mg/l as CaCO3), and of course the Perger and the 70/30 recipes, having zero calcium hardness, would not meet the spec there.
Finally, FWIW, I've recently noticed that the water spec that keno posted above, and which is linked fairly often in HB discussions, is not referenced in any of La Marzocco's machine installation guides, nor linked on any of their web sites. Recent discussions of water by the La Marzocco sites refer to a more forgiving spec (http://www.lamarzoccousa.com/wp-content ... ations.pdf
) that does not mention sulfate.
For La Marzocco on water, see:http://home.lamarzoccousa.com/water-for-home-espresso/http://global.lamarzocco.com/en/water-q ... episode-2/https://www.youtube.com/watch?v=h9AcyKNp81U
(Scott Gugliemino talk)