i also have a cabelas hawken
i have had a cabelas hawken for about 15 years now. they shoot great.. round ball works the best out of mine.. but conicals are not far behind. powerbelts are third. and sabots were just a nightmare, both in acuracy and loading. and also getting plastic fouling in the barrel. for hunting i use conicals in the 300 grain range and us about 85 grains of pyrodex. and also order a hotshot nipple if you plan on using pyrodex or trip 7. otherwise you may get some hangfires.. both of the synthetics have a higher ignition temp. and forget about using pellets . not sure if that was somthing you were looking at. now as others have stated.. only way to know what charge to use is range time.. and getting your barrel broke in. i typicaly lean twards the acuracy side of things rather than power. even if your gun likes 65 grains of powder.. thats still more than enough to bring down deer sized game out to a 100 yards.
good luck and enjoy your new rifle.. of all the BP guns i have . that 50 cabelas is still my favorite to shoot. and the one i shoot the best with.