Hi!
Maybe you should cut back on the number of escorts. HX229 (March, 1943) had 38 ships that were initially guarded by four destroyers and two corvettes - some escorts straggled or detached with survivors of sunken ships, while other escorts joined the convoy during its crossing. When the U-boats hit, it had only five escorts (three in front and one on each flank). See this URL for more details:
http://www.warsailors.com/convoys/hx229.html
Pablo