High. I mounted a 4-12x40AO and wanted to be sure it cleared. Might have gotten away with something lower. I think there's about 1/8" difference at each size increment, maybe 1/10". If you call Talley, they will help (and answer the phone!).
drover,
To make matters worse, my 453 American was 13mm. I used Leupold 13mm mediums on that IIRC.
