fellas, you are over thinking the thing. past first or third base means past FIRST or THIRD base. the foul lines run right along the edge of either bag.
no matter where the ball initially lands, if it spins back untouched and crosses into foul ground by passing over the section of the foul line that extends from the plate to the base (marked in yellow in the diagram below), it is a foul ball. if it crosses into foul ground by passing over a section of the foul line that is at or beyond first or third base (marked in green in the diagram below), it is fair.
the blue curved line is indicating a towering fly ball in the infield. let's say this ball lands where this line ends in the area between the mound and the shortstop. for some reason, nobody is able to get a glove on it. let's assume it had so much spin on it that when it finally landed it took off spinning backward and rolled, untouched, across the 3rd base line and settled in foul ground (indicated by the purple line.) this is a foul ball. the distance it traveled has nothing to do with judging fair or foul.
the only thing we use to judge fair or foul is
where the ball actually crossed the foul line. theoretically, you could move this same situation in the diagram so that the ball lands behind the shortstop in the outfield and then spins all the way back to the same spot as marked in the diagram. where did it cross the foul line? before the first or third base bag. foul ball. will it ever happen? probably not, unless, like was stated in another post, you are playing on asphalt or the old school artificial turf (not the field turf, the stuff that is carpet over cement like the old domed stadiums.)
