MARLOW: I nave a 6.5-20x50 on my 243 PSS. I use leupold medium PRW rings on a Ken Farrel mount. Leupold measure their ring hieght from top of the base to centre of the ring. Warne measure their rings from the top of the base to the inside bottom of the ring, this being .375" for standard warne rings and .410" for premiers. This plus half the tube diametre .590" plus the height of your base (?),subtracted from half your objective bell diametre 1.15" will give you agood guideline. I have a .375" gap between the bottom of my sunshade and the top of my barrel which leaves about 1/4" when take into account the flip up covers.the farrel mount is .430" nigh and the rings are .850 which is 1.280" which means i have .130" clearance from the top of the action. I hope this info will help you in choosing your rings and bases,aslong as your combination is the same or greater than half your obj bell dia. you'll be okay.