The surest way to developing a flinch and get off to a bad start accuracy wise is to start with the hyped "150 grain" load. DON'T DO IT!!! That much powder will kick your butt if you start with it.
In my opinion, any newbe should start (at the very most) with 80-90 grains powder. Get a feel for the gun. Shot it a bit. You got LOTS of time between now and next fall.
Still not convinced you don't need 150 grains? How about the 45-70 gun? Killed a heck of a lot of buffalo with 45 caliber and 70 grains of blackpowder!
Yes, trajectory of muzzleloaders is not like a 270, never has been, never will be, so don't try to make it. If you are really going to shoot animals farther than 200 yards, you better practice a lot at those ranges and have a ballistics table taped to your stock and a rangefinder in your pocket.
However, if you really are joining the ranks of muzzleloading to enjoy the sport, try your best to keep your shots reasonable muzzy distances: less than 200 yards. For that, you really don't ever need more than 90-110 grains of powder.
My elk load is a 350 gr FPB conical bullet shot with 90 grains of BH 209. More than enough energy to kill an elk out to 150 yards. Why would I want to use more powder to kill a whitetail? Not only that, but BH is expensive, and 90 grains costs a lot less than 120 grains and I definately don't flinch as much = more accuracy in the field
Loose powder and pellets are not equivalent!!!!
150 grains of pellets does NOT equal 150 grains of loose powder. 777 and Blackhorn 209 are more powerful in their loose form. They recommending DOWNLOADING by about 15% from what you use as pellets.
Last thought: loose powder is CHEAPER to shoot! Even BH 209, the most expensive powder made, is cheaper to shoot than pellets.