I usually purchase the large spare rib sections (brisket bone off) and proceed to pretty much hack them up from there.
I cut the flap off and remove the membrane. Then I flop the ribs over and from there I'm never quite certain where to cut for St.Louis cut.
Let's say I locate the longest rib and beging cutting the rack that length. Because the ribs get shorter as I go across, I would be cutting througe the rib tips. Or, I can work the knife along, cutting the ribs at the ends of the bone, removing all of the rib tips. EH? And winding up with a wide-to-narrow rack.
What do competition cook do as they prepare their ribs? Keep them more even in length, with partial rib tips on the ends of some of the bone?
As usual, I hope this makes some sense.