Compensating for tartness

I made a fruit saison that fermented from 1.048 to 1.012 cleanly. After two weeks in the primary I secondaried with a puree of 4 pounds strawberries, 2 large stalks rhubarb, and 1.5 pounds raw mesquite honey. This fermented for another week, leaving a final gravity of 1.008. Currently it is cold-crashing at 31F in a tertiary fermenter while I decide what to do with it.

As you may imagine, this beer is spectacularly tart. The fruit + additional sugars dried out the beer, and the rhubarb is *quite* noticeable. It's so sour that it almost tastes like a Flanders red ale.

SWMBO (for whose birthday this was made) has expressed dislike of the present tartness level - I am fine with it, but oh well. My plan is to kill the yeast through an extended cold-crash followed by Campden tabs. Then I will back-sweeten with raw honey. Lactose is out of the question due to thermonuclear flatulence.

My question is, how much honey should I add to 5 gallons to obtain a gain of about 10 gravity points? I'm shooting for a gravity of 1.016-1.018.

According to some brewing software, you will need about 10oz of honey per gallon of liquid to bump the gravity by 0.005 (to get you from 1.012 to 1.017).

However... I wanted to point out that campden and cold will not kill the yeast. They will simply slow the yeast down. If you add the honey and then let the stuff sit at room temp for a while, it will start fermenting again.