two tablespoons sounds like a good place to start for that volume. I'd also test every day for the first week to make sure it's still not dropping two quickly. After about a week or two you should have a pretty good idea if you need to increase it or not.
I run 1 tablespoon for my 60+20 sump for just 6 hours a day to maintain my po4 at .02-.04 (Hanna) of course that's just my system and will likely be much different for you.
On a side note I've never lost and coral running GFO but I've really ****ed it off a few times.
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