The ID of the tip top tube was 1.8mm.
I then used size 6 BLAG rings on the top section, then tapering up size, 7,8,10 and 12 but ring. These are ring sizes rather than inside diameters. My choice of rings is probably on the small size but I wanted to keep the weight down as much as possible.
If you look on this page you'll see how H&H / Fuji size their rings:
Fishing Frequently Asked Questions
I would think the sizes on the St Croix website probably equate to the snake ring sizes on this page.
Pete