It's got a tapered butt so if you fish with the reel near the bottom of the rod you'll need a 30mm. I fish reel up and found I had to really pack loads of tape under the reel-seat & it was well bulky, so I got my rod-builder to remove the butt ring and slide a 28mm down from the top which fitted much better (if you've got a 13ft model there's no butt ring anyway so this is much easier to do) then got the ring whipped back on. At the same time I had that old horrible slippery shrink tube replaced with the nice velvety Japanese stuff from Century, which may be a little thicker, but not as thick as the criss-cross stuff, so that might have an effect too depending what you've got on your rod.
Hope this helps,
Gary