They have a chart:
They have one that just peeks out past a 21" Comp with an extended choke. I had one, but cannot find it and think I gave it to a friend. Or maybe I have soooo much crap I do have it... somewhere...
Now I keep it setup for three gun with a goofy long thing on it (don't hate the playa...), but they are easy to switch. I find it helps to drop a 22 cleaning rod down through the hole in the cap and through the spring.
I do not use a clamp for many of the reasons mentioned here, mainly I had a problem kinking the transition area and stalling the follower, but also it is easier to swap the tubes. And we stand around more than we shoot, so it is easy to check.
ETA: I meant to mention Shooters Connection used to sell separate tubes without the cap and spring, making it easier, actually just cheaper, to have a couple variations, but it doesn't look like they list them anymore.
ETA: Looks like +3 is what you want for the 21" Comp: