I was in the same boat a month ago trying to get the perfect rail/rings combo with my scope(Nightforce NXS 42mm) on a Tikka. I finally just decided I would start ordering sizes from vendors that had easy simple return policies and go from there. I got lucky on attempt #1 and went with Mountain...
I just ordered one from Lenses, Digital Cameras, Passport Photos, Photography, Used Cameras, Dark Room | District Camera and Imaging. Says it will ship tomorrow so we will see.