10ppm SMB eats up 2ppm O2 per my understandingDo you know where you came up with the 3g amount? I have done this a few time, but I was not sure how much I should add. At RiteBrew, a 1 lb pack of PMB is under $6, so cost is not much of an issue.
With most of my 5 gal kegs, the dip tube angles into a small indentation in the center of the keg. These kegs leave very little solution behind. My 10L Torpedo kegs have more room from the end of the dip tube to the bottom. The dip tubes on the Torpedo kegs are already pretty short, but maybe I should try to trim them up a little more.
My Hazy IPAs seem to stay fresh for 2+ months in the keg (though they seldom last that long).
Bru'n Water has a SMB parameter and if I plug in 5.5 gallons for the volume with 150ppm that yields 3 grams
This would scrub 30ppm of DO which is plenty more than there would ever be in tap water.
If you are concerned about having more residual sanitizer than I do then you should probably cut the dose to 2 grams.
Mix the SMB in water first before adding too