I think Ken's main reason for the slow, shallow grooves, is to shoot the ITX balls just as accurately as lead. With the ITX - I'd use as thick a patch as I could and still load it. You might have to use a wad to prevent blow-by, but that was why the shallow rifling, for a tight fit without cutting, to the bottom of the grooves.
For a normal soft lead ball, I'd probably use a .495", but a .490" pure lead ball with an .018" to .022" patch should work just fine, giving a minimum of .005" compression on the bottom of the grooves. That would be a minimum load, as I've found .005" has been required to prevent cutting and gas cutting - along with accuracy. Due to the shallow rlfing and slow twist, .005" compression (or less, perhaps) might be all that's necessary, even with heavy loads.
As to powder charges, shoot what gives the best accuracy, however, I can say with complete conviction, that, that barrel will shoot more accurately with any increase in powder you are willing to pour down the tube- to perhaps 130gr. 2F or maybe even more. Note that as the powder charge goes up, so does the pressure. Increased pressure demands increased compression of the patch and lead in the bottom of the grooves - ie; higher the load, the tighter the ball/patch combination has to be.
In normal faster twist and deeper grooved barrels, if you don't want to use a tight combination, there will be a ceiling where any more powder will cause gas cutting - at that temperature and atmospheric condition. This same condition may happen in your new barrel, Ken, but to a lesser degree. Ie: the ceiling might be 80gr. with one combination, and 120gr. with another. Note I said at that temperature. This is more important than we realize and I've proved to myself it happens more often that we realize. A load that works in spring or fall, can fail in 90F or 100F temps. It happened in my .69 as well as my .40, and I usually shoot tighter combinations than most other shooters to start with.