I discovered a cool new feature that was stealthed into the LI interface during the book 7 patch. (I never heard about it and didn't see it mentioned in the patch notes.
It's called "track" and it is a small radio button located just to the right of the XP bar for each of your legendaries. By clicking on it you will bring up a tiny XP window on the left hand side of your screen that will let you monitor how close you are to leveling that legendary. You can bring up any or all 6 if you want to and the button toggles so clicking it again will kill the display.
Pretty cool, eh?