figuring out the right size for a tube jacket

I’m seriously weighing up the tube jacket. I feel a 0g (8mm) lock would be ideal for me. Back in September, my piercing was done at 6g and it has shrunk down to size 2 now, so after one more stretching session I’ll be ready to place an order. My current query is: what’s the best method to measure the inner diameter of a tube jacket? Should you do it while flaccid, fully erect, or somewhere in between?? I’d really appreciate any feedback from people who have used the Steelworxx Tube Jacket or other similar Loris-style devices.