I don't use paraffin so I'm not sure how much help this will be, but it sounds almost like your FO is settling below that quarter inch mark as the candle cures. Unless of course, paraffin is supposed to have a full melt pool in .5 x jar diameter? If that's not the case I'd say your wick is burning the FO off somehow on the first burn but not on subsequent burns. Not sure why on that, though, unless it has something to do with the cooler burning of zinc wicks. How tall is the wick on burn #1?