For clamps always use the outside diameter of the hose, the surface that the clamp is actually in contact with.
As you say the T-bolt clamps don't have a very wide size range compared to the worm gear style. But, they are vastly superior for providing an even clamp pressure around the entire perimeter. It is better to error on the side of the clamp being slightly small rather than too big. If the clamp is on the small side you can remove the nut and open the clamp up larger then gently squeeze it together to get the nut started again for tightening. On the other hand once the nut bottoms out the clamp is as tight as it can ever be so you don't want it oversized.