this does work, of course it does not if you have most verticals and have to cut the ribs in two to fit.
Ewan,
221, 321, 211, etc are meant as guidelines for ribs (Spare or BBs) at 225 degrees. Sometimes the ribs cook quicker, sometimes they cook slower. Do the "bend test" on ribs to check and see if/when they are ready.
http://www.smokingmeatforums.com/t/165426/bend-test