If you have tight miters and the tubes are properly prepped and its brazed well, 56% will be strong enough, IMO. However, fillet brazing with LFB is a little more forgiving of technique and would be stronger. As unterhausen says, heat sinks help prevent distortion of the seat tube. There are many different kinds from simple steel "plugs" to fancy expanding brass ones. I use the steel one shown below. If using steel, be sure to pull it out while the weld is still hot, or you might never get it out. FYI....with brass and a heat sink you may still get enough distortion that you may have to ream the seat tube to get the seat post back in.
Dave
