I also use Pavlis' simple water recipe in all of my machines and for cooking, as well. I add the potassium bicarbonate to RO water.
For minerals, I spray them onto my pucks before extraction-- this way I don't have to worry about any effects in the boiler/plumbing sections of my machines.

Spraying mineral solutions on to pucks-a facile way to contour water minerals on a per cup basis

Now this minerals-on-puck approach very much interests me. Mostly in the ability to customize water makeup for extraction of each coffee individually. The only concern I have is how it extracts with it all waiting on the surface versus mixed in the whole body of water.

A very good question. My guess is that it will be different, but it may prove equivalent in the cup. ??? Since I don't add minerals to my machines, I can't do a direct comparison. But the neat advantage of this approach is that you can vary the blend and amount of minerals on a per cup basis-- aside from obviating any boiler/effects.
(Easy to imagine gradient concentration effects, and so on, which may prove beneficial, or deleterious ????)

I started with the BH mag+na recipe. I found I had to make a really low TDS (based on those solubles) to get water that didn't make the espresso really sour.

Then I found the 10g Potassium Bicarb to 100ml DI water - then 4-6ml of that to 1 gallon of DI water -- and it totally changed my shots and I never looked back again.

I'm curuos about adding calcium but no big rush. This recipe is allowing me to forget about water and focus on extraction.

I'm about to get a new tank-filled espresso machine and have been reading literally all of the information about water here and elsewhere. I'm trying to pick one method and never have to think about it again. At this point I'm inclined to go with the rpavlis method for the simplicity, lack of scale in the machine, and low cost. I think it's that or Third Wave Water, although from reading his posts it seems he never felt the Ca and Mg in the water was necessary.

My only question at the moment is whether the simple distilled water plus potassium bicarbonate causes any issues with the machine sensors. I know some of them need mineral content in the water to detect fill levels and possibly heat and pressure; does the potassium bicarbonate do the job, or is Ca/Mg required for that?

Adding the sodium or potassium to the distilled water makes it sufficiently conductive all of the electronics should function. Just tested it with a Fluke 87 and it conducts just fine. Keep in mind buying distilled water from Culligan is lower cost than the grocery store. Potassium bicarbonate can be purchased here: https://nuts.com/cookingbaking/leavener ... onate.html among other places.

Thanks for testing that. I already bought some potassium bicarbonate and tested it in one gallon of distilled water, brewing moka pot coffee and making tea. Seemed fine.

I don't seem to be able to get Culligan delivery here. I did research Mountain Valley delivery, but at $2.59/gallon (+tax) it was significantly more expensive than the grocery store. Most of the water delivery services require you give them contact info to get a quote, which is irritating. I also looked at those countertop water distillers, but am unsure if there's any cost saving by the time you've spent $240 on that and have to power it.

Keep an eye on Craigslist as distillers pop up rather regularly. If you go to the local distilled water company office they sell 5 gallon jugs of distilled water for about 6-7$ ea and you get to keep the jug. I buy them from the local Culligan when I run out of jugs. I have a high output still so I run it for a week or so in November and make a years worth of water for coffee.

but what does it distill the other 51 weeks a year?

You can also do countertop RO and not need to power it. There is considerable waste water, but that's a lot cheaper than the electricity it'd take to run a distiller for me.