One very overlooked part about dryed out meat is the lack of preparation going into the cooking process. Season it well at least a day beforehand. Brining will help moisture retention, and will resist drying out much better. Use a kitchen scale to verify your meat weight. Then weigh out your salt and brown sugar. The sugar doesn't sweeten the meat, but is a key part of the brining process. No need to submerge for a wet brine. Just mix your salt, sugar, & seasonings and apply to the whole roast. Flip it over a few times in a day or so to ensure good contact, and cover in plastic wrap so that the extracted liquid doesn't evaporate.
(1.5% salt) = meat weight x 0.015
(1.0% brown sugar) = meat weight x 0.01
For your weights, it's easiest to work in Grams vs. Pounds/Ounces. Garlic powder and black pepper will be a good addition to any brine as well, so use about 10g in your brine.