Each muzzleloader shoots differently. That is, you can put identical powder/bullet loads into two muzzleloader rifles and get different results in terms of energy and accuracy. So, how does a shooter know what load is best for his rifle? He must learn this through simple trial-and-error testing on the range.
Start by deciding what powder/bullet combination you'll be using to hunt. Then start experimenting with different powder charges to see how they affect bullet flight. Set your target at hunting range (75 or 100 yards), and shoot 3 shot groups with different combinations of powder/bullets. I prefer using Pyrodex pellets, and I'll use different bullet weights with 2 and 3 50-grain pellets. Shooters using loose powder could increase loads in smaller increments, say 10 grains of powder at a time.
Keep accurate records of results of each combination, and soon you will learn what load shoots best in your muzzleloader. This testing process takes time and some expense in terms of using different bullets. But once you determine which load your muzzleloader shoots best, you can stick with it over the seasons and shoot with confidence that you're getting the best performance possible.