IF your stadia sight is calibrated the way I think it is, each mark is the tangent of 1/4 of a degree (each whole number being one degree). The gunner called off the range, the people at the limber chest called back the elevation needed for that range, extrapolating as needed. The gunner then set either the upper or lower peep at the mark needed. He then aimed the piece.
Just pulling numbers out of the air - say the gunner calls out "Counter Battery, Shell, Range - 1500 yards." Back at the limber they would check the Table of Fire, call back "Shell, 1500 yards, 3 7/8 degrees, 4 3/4 second fuse" The gunner would set his sight for 3 7/8 degrees and get his piece aimed, the guys at the limber box would cut a bit off a 5 second fuse (they came with cut marks printed on them) and put it into the shell. Round comes forward, is loaded, final check on the aim in case the gun moved during loading, prick & prime, fire. Then get ready to do it again and make any adjustments needed.
Oh, about that taper - I think that is so that the slider can't be pulled off the top of the post. The ones I have had my hands on, that slide can only come off by sliding it down.
And about the powder, remember that using Fg will give you somewhat higher pressure than Cannon grade powder.