Eric,
Your Huntsman should have a 24" barrel. Remember that the 85 grain load of 777 is equal to 100 grain load of Pyrodex as far as energy. 777 VOLUME charges should be reduced 15-20% compared to Pyrodex or Black powder by VOLUME.
When bore sighting, the height of your scope can play a part in how close you will get. The higher your scope, the further you will want to start. Just remember, bore sighting is not an exact science and is only to get you on paper.
EYEBALL METHOD.
Most people pick a round object between 25 and 50 yards and sight down the barrel (breech plug removed) with the eyeball, centering the round object in the bore. While the barrel is stationary, turn your crosshairs to the center of the round object (ball works good). I actually adjust the crosshairs to just a little below center when the object is at about 50 yards. This should get you on the paper and you will have to adjust from there.
You won't need a lazer with this method.
If you sight that load in at 100 yards by shooting you should be good to go at any range out to that point. The height of your scope will determine your closer impact points, but the difference will probably be minimal. Just shoot at any range you plan on shooting. Might want to shoot at 50, then move to 100 for final sight in.