Your tapwater's pH 4.0??? Where do you live--the gates of Hell?

No, wait....that'd be me..... Hermie lives at the Gates of Hell...

If there's a "flushing standard" it's probably 3 gals flush for each gal of pot volume = 9 gallons for a 3 gal pot.

Or, do it the easy way and catch some of the runoff next time you water. Keep watering, and periodically measure the pH and ppm of the runoff as the numbers improve (ppm v , pH ^). When you get the ppm / pH where you want it, you're done! The Perfect Flush. :thumbsup:

I do the last runthrough with a light nute solution as Farmer Rich mentioned. After all, if the nutes have been locked out they've been unusable...

And if you're flushing with RO (or especially distilled) water, remember it's gonna leach out minerals, so you should add calmag on the last rinse or two. (IMO)

I need something to get it around the mid 6 range
That Promix BX is basically peat (acidic) that's been temporarily tamed with lime. It runs 5.5-6.0 like that. If you've exhausted your lime, the peat's gonna start dropping the pH. But if you can get your pH in that 5.5-6 range you should be fine. Just get a little "pH Up" of some kind.

And check the calibration on all your meters. :thumbsup:
DreadedHermie Reviewed by DreadedHermie on . Need Guidance on Ph issue Quoted from Site Adjusting pH Of Marijuana Grown In Soil A good way to stabilize soil is to use dolomite lime (calcium-magnesium carbonate). Dolomitic lime acts slowly and continuously, so soil will remain pH stable for a few months. Using fine size dolomite lime is important, coarser grades can take a year or longer to work. You can find fine size dolomite lime at any well stocked garden supply center. Dolomite lime has been used by gardeners as a pH stabilizer for many years. It Rating: 5