How much baking soda? Hard to say without a KH test kit. Since your fishless cyling, though, you don't have to sweat it too much. Just add a tablespoon every 4-6 days. That should be more than plenty. (Another advantage of fishless cyling: if you had fish in there, you'd have to worry about the sudden jump in KH.)
Have you changed enough water to get nitrite down to a level where it's not maxing out your test? You are still adding ammonia, right?
Jim