Been following this forum for a while and the Barista Hustle & RPavlis recipe are thrown around a lot, for a good reason of course.

As a background, this is 2 weeks old Rocket Appartamento HX.

The main consensus (that I observe) is that RPavlis user can theoretically run forever without descaling due to 0 hardness & 50 ppm alkalinity.

Now if I were to play around with GH mix (Epsom salt concentrate) and produce following formula:
90ppm Hardness (Magnesium from Epsom Salt)
50ppm Alkalinity (Sodium Bicarbonate/Baking Soda)
Source is RO water.

Question:

Is descaling necessary with this custom recipe water (90ppm GH & 50ppm KH)?
Do I need to adjust backflushing routine for longer to avoid squeaking lever? (once a month perhaps? Rocket recommend once every 2 weeks)

Thanks again for helping out on this.

No need to descale. Limescale is calcium carbonate and your recipe has no calcium. At high concentrations and pH you could theoretically get magnesium hydroxide or carbonate scale but not likely at this level.

The added magnesium in the Epsom may (or may not) improve the taste, and the added sulfate may even detract from the taste. The sulfate may also increase corrosivity. Natural water and typical treated waters don't have much magnesium sulfate, so adding it gives you a water different than what most espresso shops and roasteries would be using. For that reason I'd recommend trying a recipe with little or no Epsom and using that if your espresso tastes fine.

Re backfushing I'd say just stick with a no-detergent daily backflush, and occasionally remove and clean your shower screen with detergent. If it tends to be dirty when you do that, then you can do it more often. Then maybe once a month do the detergent backflush. If you follow that with a good rinse and a shot of coffee I don't think you'll hear any squeaking. If you do it's easy to pull the lever and lubricate the cams and seals.