It's not that I have an abundant amount of free time or try to push the BBB method on anyone but when they ask, I try to explain. I've been helping a friend straighten up his pool after he admired how "sparkling" my water was. Week before last, here is what I discovered:

21k gallon A/G vinyl
FC .5
CC 1
pH >6.8
ALK 0
CH 0
CYA <100
Algae was present around the seam throughout most of the perimeter (and probably under steps), water semi cloudy but the bottom was visible.

I started the whole "education" by having him read the FAQ's at Pool Solutions, Downloading Michael's Bleach Calc and Ben's Best Guess Chart to his desktop and signing him up at Trouble Free Pool so he could post there if he'd like. He ordered Dave's (Duraleigh's) test kit. I told him he had two problems (low pH and High CYA) and I'd attack them both at the same time by draining half the water then refilling. He was a little hesitant and wanted to attack the low pH first. He ended up adding 4 boxes of Borax and it didn't even touch the pH. He then decided to try it my way and drain half and refill. He then added 4 MORE boxes of Borax and here is what he's got:

FC 9
CC 0
pH 7.5
Alk 80
CH 0
CYA <100

He's adding baking soda tonight to creep up to 100 ppm of ALK and keeping on top of the FC and we'll dilute the CYA sample tonight to try and get a guestimate of where it is.

Now, to the jest of my post:
21k gallons + 4 boxes (76 oz each) Borax - 10.5k gallons of water + 10.5k gallons of water @ pH 7.2 (fill water) + 4 boxes (76 oz each) Borax = 7.5 pH.
How low could his pH have been?

Dave