A lot of folks are over complicating this, in my opinion. I have used the Thor sizing pack for my Traditions and it was a cinch that needed absolutely no extra tools, other than a mallet.
Take your rifle to the range. Load your powder charge & the smallest diameter bullet in the pack, preferably with your field rod because that's what you have in the field. But make sure you have a sturdy range rod too. Now, did it slip down the bore with minimal effort? Or was it a pretty tight fit? If it's tight, you're done. Shoot that bullet out and order more of that size. Now you just need to see if it's accurate -- next range trip with your order of bullets, you can find out.
But if it slipped down the bore with minimal effort, then swab the bore fairly clean, put in a fresh powder charge & try the next size up. When you need a mallet or just a lot of force on a range rod to ram that bullet down on top of the powder, then you went too far. Order the last, smaller size. There's only 4 or 5 bullets in the pack so this won't take very long. If you want to be extra safe, put a coat of some kind of grease on the bullets or a greased patch to swab the bore before loading so that you can ram it the rest of the way down with a mallet if needed & then shoot it out.
Mine load fairly easy but not too easy. I can barely get it started with my thumb but still use the short starter to get it going down a few inches. The skirt will expand the rest of the way to fit the bore, just like some plastic-skirted ones you find for sale at most stores. In fact, the Thor 247 gr .501 load & shoot almost exactly like a plastic-skirted 295 gr popular bullet I find just about everywhere (forum rules, I won't mention the name of it). Both have been VERY accurate for me.
The Thors are not meant to be shot from fast-twist in-lines although they can be. They are perfect for 1:48 twist rifles because they're fairly short for their diameter, as copper goes. Much better than a sabot bullet, which is often a bullet 0.05" less in diameter for the same weight (.50 sabot has .45 bullet). Those need faster twists, to be sure. I haven't ever shot those in my guns & don't plan to.
About a year or two after I did my sizing pack trials, Thor came out with a "one-size" bullet that has a split skirt and doesn't require the use of a sizing pack. It's meant to fit all bores around that diameter & expand to match the bore it's in. I haven't tried these but it might be worth seeing if those work before going through the sizing pack setup.
One caveat about Thors is that they need a lot of speed to open up. A 90 gr load of Pyrodex RS gets me right about 1600 fps at the muzzle with Thor 247 gr bullets. At about 60-70 yards, it did not expand on a deer I shot in 2020. I forget what kind of speed the bullet would have at that distance but it was supposed to still be within the expansion window, according to Thor. I had a brief email exchange with Thor about this issue and they assured me that their bullets would open up. But it didn't so... At 10 yards (follow up), it expanded beautifully. Again, speed is what you need. Treat it like a pistol bullet -- speed gets you expansion but limits penetration. Lower speeds may not expand but you'll probably get more penetration. At .50, it's lethal either way so I wouldn't sweat it too much. But I didn't think I was gaining much over a lead free roundball like bismuth or ITX that also won't expand. So pick your poison. I'm a cheapskate so I just started casting my own & gave up using Thors for now.