I have done a few hundred of these flares and a few things I have learned. x2 on the seam. That is creating part of your problem. You need seamless tubing. Also you need to make sure the tubing you are using is annealed, it will not flare well if it is too hard. Also the flaring tool that you linked to is not the best for doing the 37° flares. Mcmaster Carr has a very good one that I use but it is a little on the expensive side, about $160. Tube Service is a good source for the tubing, I have purchased this material from them before. Also make sure you don't go any thicker on the wall. Hope this helps.
Kevin