Your T/C Hawkin more than likely has a 1:48 twist. This is a "middle-of-the-road" twist that will shoot both round balls and conicals well, but not exceptional. I have 3 of these fine rifles in flintlock. In PA where I live, up until a few years ago you could only use a round ball. Since then, any projectile is legal. I find that one shoots the Lee REAL bullet (250 gr) well, while the other prefers T/C 275 gr Maxi-Hunters. The last rifle, I purchased a Green Mountain Long Range Hunter barrel with a 1:28 twist and shoot 245 gr Powerbelts out of it.
Be forewarned about using too light a load. IMO and bad experience, 65 gr of FFFg is too light for deer. In my early ML years, I shot one of the biggest buck I ever saw in the woods (and that's a lot) with a patched round ball and 70 gr of FFFg. My best guess is that the shot was a bit too far forward and smacked into the shoulder. I knocked every loose hair off that deer, or so it appeared in the snow. Tracked him for 1/2 a mile and only 3 or 4 drops of blood. Never did get him. With a 50 cal, I would start with 80 grs. and work up from there to 100 grs. More than 100 grs, your just blowing unburned powder out the barrel. Since then, I have taken about 6 deer with my MLs. I just love being out there in the dead of winter. Alone with nature and your thoughts. Have fun.