I always found finish ream size is hit or miss. A new ream might cut oversize. A dull one might cut even more oversize. Run at one RPM and feed and get one result. Change RPM or feed and get a different result. Milling it might get you a tapered hole. Want a guarantee? There is not one. Personally, I would get one 0.249 ream and two 0.2495 reams. Then I would try them and find the right combination of tool, RPM, and feed.