when your OG gravity makes it not worth fermenting to drink? but still then could be good kombucha!!!
I've just stocked up on DME for that.
Brew shops are not going to change their mill gap. Expecting or even asking that is a pipe dream. Some won't even run it through more than once.
The 70-72% mash efficiency you get with barley is already an indication something is off with the milling.
Either start looking for your own adjustable mill or learn how to deal with their overly coarse crush.
If you can buy bags of base malt directly from the maltster and mill your own batches, you may save enough money that repays tour mill investment. Most of us use 2 roller mills, running US$100-200.
A knock off Corona mill may run US$25 (e.g., AliExpress) and do a wonderful job with the right adjustments. Use a drill to motorize, or some other motor.
The brew shop owner says he does mill rye finer that wheat and barley. I don't know what gap but I will ask.
Use mash water calculator to estimate acid additions. I like BrunWater. Free edition is fine.
You really should measure mash pH at the end of the mash to get a valid reading. Make sure you get a representative sample. And chill it to your pH meter calibration temp or thereabout.
Or do a small 1/2 - 1 pound test mash of your grist mixture (measure each ingredient accurately), to get the real pH. Adjust your main batch acid additions accordingly. And add that test mash to your main mash, at the end.
No wort left behind!
I've set up the EZ water calculator with the water report info from the local council.