Well this is great! Thanks to the advice I have read here, I just finished adding borates to my spa. Pool Calculator came up with 12 oz by volume of Borax and 6 oz of muriatic acid.
Not having dealt with full-strength MA before, I took precautions: chemical resistant gloves, safety glasses, a deep breath, a muppet assistant and a white lab coat (kidding, but maybe next time to freak out my neighbors).
First, I added half of the recommended MA to the tub, so 3 oz, with the jets going full blast. I then mixed up half the recommended amount of borax and hot spa water in a 5 gallon bucket. Before adding it, I noticed how there was now a bit more foaming... perhaps a combination of the now low pH and my naturally soft Cascade-fresh water (you're soaking in it, Madge). Anyway, adding the borax solution quickly put the foaming back to normal levels. Interesting!
Rinsed, repeat. Same foam and then no-foam results. Waited a bit for it all to mix and tested the pH. A bit high, dinked around adding a touch more MA, then a touch more borax (uh oh, am I chasing the numbers?), and I'm back at the 7.5 we started with. I'll check it again before soaking tonight. I think 215 gallons mixes rapidly with the jets on, as evidenced by the appearance and disappearance of the foam.
The MA was easy to deal with... if it fumed, I didn't notice as I held my breath while quickly dealing with it. I rinsed everything off in the circulating spa water, including the entire bottle of MA, which is now stored in its own 5 gallon bucket in the basement corner away from anything vulnerable. Maybe being over-cautious, but lots of reading here convinced me to be on the safe side of the line.
Borate test strip shows somewhere around 50 ppm, which was what I was aiming for.