I asked that very question here and the opinion of most here is that it doesn't matter; last groove or middle groove. I would say one advantage of the middle groove would be if you need to get a lever in to pop out the wrist seal from the suit, you may be less likely to damage the seal. But then if you're popping out the seal then it's probably damaged anyway.I just finished installing the Rolock 90 kit. Glove of choice is a Showa 720 11/XL nitrile and the yellow 10/L liners from a 495 set. Easy peasy. The glove rings rotate and lock/unlock without problem using the same hand twist technique as shown in the Rolock 90 vids.
During the install/switch over, one thing I noted was how Seaskin installed the Sitech silicone wrist seals on the QCS Oval rings, with the seal's edge lip placed all the way to the last groove, not to the 2nd grove as shown in the Rolock vids. I confirmed the edge/lip placement in an older Seaskin video as well... all the way to the last groove.
The Sitech manual provided with the suit delivery, explains the proper groove to use depends on how the suit mfr installs the permanent ring into the suit. Since Seaskin preinstalled the seals in the last groove, I would assume that as the proper setup to use when installing the Rolock rings (groove system appears to be identical to the Sitech QCS Oval).
So where did you source your Rolocks?