It's not just seasoning - the salt draws the moisture out near the surface, which means a) that nice nearly-burnt flavour at the edges, and b) the moisture further in tends to stay there. At least, that's my experience.
As for the thermometer...use one with a proper oven-safe probe, and it'll tell you when it's done. For beef joints, I cook mine to 51C at the thickest part. Put in a dish and cover with foil, leave to rest for about an hour (after pouring over any cooking juices that came out), then slice thinly with a non-serrated knife (you'll need a good knife). It continues cooking during the resting time, and by the time you serve it will be a perfect pink in the middle.
It will be a bit cooler by then, but if you slice thinly then the gravy (you are serving the gravy piping hot, right?) will heat it up.
Oh, and obviously use the resting juices in the gravy. If you've seasoned the beef properly, the resting juices will incorporate some of that seasoning, and thus season your gravy too. Ergo, don't add any seasoning to the gravy until after this point.
My personal favourite way to cook beef is to go nuts with the pepper prior to cooking. You end up with a lovely peppery outside and slightly piquant gravy to boot.