Two methods -

First,
Shoot the rifle at tall targets set up a various distances to 300 yards.

Keep it up until you know how the rifle shoots.

Second,
Measure the muzzle velocity at four or five ambient temperatures from cold to hot.

Calculate the ballistic coefficient of the round ball you're shooting.

Enter the info you have into a ballistic trajectory calculator such as at the Hornady web site.

Here's an online calculator for muzzleloaders -

https://www.ctmuzzleloaders.com/ctml_experiments/rbballistics/web_apps/rb_ballistics.html

Then look at the points of impact at various ranges.  At extreme ranges run the program for three ranges that are similar, say 25 to 50 yards apart.

Then get to the range to confirm and tweak the results.  The first method is the only way to get around chronographing the speeds

Simple. (It really is simple, but requires a little work.)