You're not doing much wrong actually.
But here are some tips to help you avoid dry pork. Roasting in a steamy environment won't help you much. You don't need all that water, nor do you need the tent. I suggest roasting over a pan with just enough liquid to protect the meat juices which drop into it. Or better yet, filled with some aromatics.
The biggest problem in terms of what you're doing is your temperature, which is neither here nor there for anything as lean as modern, American pork loin. Your little roast is too small to go into a hot oven, and then drop the temperature. It would be cooked before the temp slowed down much, defeating the purpose. You'd be better served by choosing "low and slow," or regular. I think regular, which is 350, would be the easiest -- all things considered.
The purpose of starting hot, then going slow is to get a sear on the surface, then coast gently into doneness. You really ought to sear a piece that small before roasting -- but it's not absolutely necessary.
Alternatively, the loin could be cooked low and slow; but that's a different thing and not very good for cooking to the medium/medium-well range you're going for.
One of the most common ways to prevent dryness in pork is to "bard" it. "Barding" means draping something fatty over the surface. You'd probably be best served by bacon.
Something similar, which might be more to your liking is a "wet rub." You said you roasted your little piece of meat unseasoned. Mistake. Proper seasoning will form a "bark" on the meat which will make it juicier.
You can make a nice wet rub my putting half an apple and half an onion in the processor (or blender), a little bit of fresh ginger, a couple of cloves of garlic, a couple of tbs of brown sugar; salt, pepper, thyme, sage and paprika to taste; and just enough extra virgin olive oil (couple of tbs) to form a stiff paste. Rub it onto the meat before putting it on the rack.
A dry rub is whatever dry seasonings you like -- could be just salt and pepper or could be a lot more complicated. What makes it a "rub" instead of just seasoning is beginning with a "slather." Most people just use a little oil. Mayonnaise works very well. So, surprisingly does ordinary baseball mustard -- which doesn't leave nearly as much taste as you think it might (because it mostly tastes like vinegar to begin with).
Oil,salt and pepper are near ideal seasonings for a pork roast.
This whole discussion begs the question of why you would cook unseasoned to begin with. WTF? I'd really like to hear your reasoning.
Get into the habit of trussing roasts before cooking them. It makes a difference.
Let's talk about what you're not doing.
One thing you can do along with some or all of the things already mentioned is brining. I routinely brine any piece of pork small enough to handle conveniently. Here's one good, easy brine that should give you the idea. Add 1/4 cup of (not iodized) ordinary table salt (or 1/3 cup of Diamond kosher) and 1/3 of sugar and 1/2 an onion (cut coarse) to a quart of water. Heat and stir only until the salt and sugar dissolve -- no need to boil. Add a pint of apple or white grape juice, 3/4 cup cider vinegar, and enough ice to chill (an amount made from a pint of water). Submerge your pork in the brine and let it sit for four hours to overnight (It takes a brine this strong about 45 minutes per inch of meat to penetrate. If you taste the brine -- always a good idea -- it should not taste good. It should taste unpleasantly salty, sweet, sour and watery -- all at the same time.
There are of course a variety of brine recipes. Each will add a slightly different flavor profile, and each will make your roast far juicier.
Better than brining, easier and surer too, is injecting.
As to final temperature. Since trichinosis is no longer a problem it's become fashionalbe to eat pork progressively less well done. However, most people do not like any rareness at all in pork. An acceptable compromise is at 150F - 155F. I don't think your temperature was a problem, but if you can convince your family to eat slightly less cooked pork, you'll get a slightly juicier product.