If you are planing to learn how to use a thumb ring, I would advise you to go no higher than the mid-30# range. I'm sure that you are able to draw much higher poundage. I am also sure that you will wind up with sore tendons in your thumb and that it will take you far longer than necessary to learn how to use the thumb draw. A properly fitting ring is a must, or possibly one of the thumb draw gloves that "Three Rivers" sells. The best bet for rings is to learn how to make your own. Final thought; trim your thumb nail short, or the bow string will do it for you.
Bob