I may be unclear regarding what part you are actually talking about. For the hundreds of rivets along the stringers and longerons and ribs that connect the skin to the entire tailcone, I back riveted 99% of them. For the bottom, lay it flat on something and backrivet. For the sides, I rocked the tailcone 90 degrees and back riveted. Even for the curves, strategically placed blocks of wood and styrofoam let me brace the tailcone at just the right angle to back rivet. Every different position required reorganizing the blocks of plywood and foam so that I was always keeping the factory heads flat and flush, but once it was set up, backriveting was the way to go. With a long bucking bar, I could bang out blocks of 10 at a time.
8 or 9 out of 10 was my score for the tail-fuselage joint using the pile of wood. For the backrivets for the entire tailcone, I was running more like 49 out of 50. As long as you triple check that the factory head is centered on the bucking bar and the metal is tight and flat, it is really hard to screw up a back rivet.