In my experience, the most common point for leaking is the bottom seal, as it's at a wear point. If the bush is worn the rocker shaft will wiggle and that accelerates the wear. The end cover can leak as well, although properly set up it shouldn't; same with the compression fitting (olive). The top cover won't leak, unless it's warped, with a good gasket and, if you like, your favorite gasket sealant (Permatex Aviation Form-A-Gasket seems to be the choice of the pros).
If you fill to the bottom of the filler hole, you still have plenty of room for heat expansion: there is no seal so the oil can expand up the steering shaft tube. Confession: I was filling a dry box once with the front-end elevated and put almost a quart into it before I realized I was filling up the steering tube.