Help with choosing Birdlocked Pico ring size

[8en]

I want to know a few things about the Birdlocked Pico.

1. for my CB6000 I use the 1 7/8th ring size this would mean I would want to get the 50 ring size however when i measure myself I am on the smaller size of their chart (152mm) should I get the smaller size 45?

2 Do the silicone rings stretch just enough that i could fit in a 45? or should i not try this? I know the locking mechanism prevents stretching but I would only need a little

Any help would be appreciated, thanks

[8en]

does anyone own both?