I ended up talking to a couple of butchers and switched gears a bit on my custom burger blend. i dry aged a sirloin for 40 days, trimmed the pellicle and chunked the sirloin and mixed it with equal amounts of brisket and chuck. The pellicle was soaked overnight in beef broth and ground in with the sirloin, brisket and chuck. The resulting blend is really good. Great beefy flavor and about 20% fat. If you were thinking about creating your own blend, I can highly recommend this one.