I use the Lee powder measures in different configurations, but I don't use Long Shot, so I can't offer any specific comments on that powder. Like the others, though, I have found the disks to simply need to be selected by trial-and-error once you use the Lee specifications to get within the ballpark.
But there is more to using the Lee Measures--or any measure, I think--than just selecting the proper disk size for the powder amount desired. You need to be sure the measure operates consistently, and that it, along with the die, is clean and operating freely, that the disk / bar is not hanging up.
Once you have that disk size / charge bar set up for a satisfactory charge, confirm that charge is consistently delivered by running at least ten test drops once the hopper has been operated enough to settle the powder. And, don't let the hopper drop below 50% full until you know how the measure feeds the powder.
Regardless of using disks or the charge bar (which I now exclusively use, or an aftermarket one I got years ago), I also have a rhythm to my charging, and that includes a standard 'rap-tap' on the hopper as well as the same kind and velocity to my push-pull on the press handle.
Finally, if you are running without the positive-return rod / chain assembly and relying on the spring return (as on a turret), be sure the PTD is set up correctly to operate the measure slide fully.
Jim H.