2 - 1/4 cups for me, I boil a gallon of RO/DI in the kettle then pour half of that into a Pyrex bowl and add the baked baking soda very slowly. Once added together I add the other half gallon to the slurry. I use a whisk to mix it in as I go. The key is to add very small amounts at a time whilst mixing. Too much at a time and it clumps. And it's not so much a clumping issue, and even though it's not really a precipitate either, but it's stuck somewhere in the middle. Ie., once clumped it will never dissolve. I sometimes get all ticked and then mix it in a blender and I seem to get a slightly higher yield but the vast majority ends up as a undissolvable supernatant still (just pulverized into powder).
I have about a 75% success ratio nowadays when I mix the stuff ("success" meaning "no supernatant") but it boils down to how patient I am at the time. They key is going slow, ..... but the problem is I hate how long it takes.
