I do the low to high cooking with a pork loin. A 5 pounder would probably go 2 1/2 hours at 300F then finish at 450F. A lot of roasts have a certain shape that allows a minutes per pound calculation but larger pork loins tend to be longer without being any thicker so a thermometer is the only way to get it right. Slow cooking is needed for most of the time to keep it tender.
My first probe got fucked up. You can replace it online, cheap. Well worth doing, IMO.
I’d say that for such a finicky piece of meat, a leave-in probe thermometer is vital. Since the meat has almost no fat or connective tissue, that means that it’s unusually sensitive to overcooking (gets dry and tough) and will probably dry up into something godawful if you cook it low and slow, since there’s not much connective tissue to change into gelatin.
Personally, I’d go with pulykamell’s advice. That’s probably the most sound way to cook a boneless pork loin roast.
I’ve got 2 other probe thermometers and a Thermopen.
This worked very well, thanks. Perfectly cooked and very juicy. It shredded nicely, which will lend itself well to making pulled pork sandwiches with the leftovers.
I’m happy to have been of help! 
I was in a hurry so I just used salt and pepper, but the long and low method worked very, very well. I did it exactly as you suggested, and it was the tenderest pork roast I have ever cooked. It shredded beautifully for pulled pork sandwiches. The texture was completely different than any other pork roast I have ever cooked.
Try the same with pork shoulder (aka “butt/Boston butt”) next time and be amazed!
I just wanted to add. Tonight I bought a 5 lb pork loin for 1.49 a lb. I trimed the silverskin and cooked it at 500 for 15 minutes and 325 until it hit an internal temp of 138. I foil tented it until it hit 145 and then pulled the foil. At 1.49 a lb, the pork is better than any 8 dollar a lb beef steak, just amazingly tender and juicy. If you haven’t experimented with the new relaxed “rules” on pork temp, try it out. I bet pork loin is going to hit 5 dollars a lb in a few years, as I’ll have a few dinners and great lunch meat for the week for 8 bucks. I seasoned it with salt, italian seasoning, onion powder, garlic and olive oil, but it would be great with anything
Today I’m cooking a picnic roast. First time I’ve cooked a piggy roast with skin on, so I scored the skin before seasoning. I used kosher salt and the pork rub I made. I also put on a little Stubb’s liquid smoke. It’s nearly 7 pounds, and we’d like to eat before bedtime, so I’m cooking it at 235ºF instead of 225º this time. It went in around 0800, so it’s been in there 6½ hours so far. I’ve mixed up some coleslaw, and bought potato buns.
Hmm. Your low temp isn’t going to do the skin any favours. If I were you, when you take it out, pull off the skin - try to keep it in one piece. Season MORE (you need lots) and broil it on high until it’s puffed up and golden.
Roast pork NEEDS crackling.
Oh, I agree crispy pig skin is delicious. But this came out just fine. Not crispy, but tasty.
In case anyone is interested, I found the pork rub I use. It’s in an older thread.
½ cup brown sugar
1 tablespoon coarsely ground black pepper
1 tablespoon kosher salt
1½ teaspoons garlic powder
1 teaspoon dry mustard powder
½ teaspoon ground ginger
½ teaspoon onion powder
½ teaspoon red pepper flakes
½ teaspoon cayenne pepper
½ teaspoon ground cumin
½ teaspoon smoked paprika
¼ teaspoon dried thyme leaves
Reported.