By all means, listen to what BrownBear is saying. You asked a specific question, and we gave you a specific answer to that question. But in working on loads, find out what your gun likes to shoot.
A lot of members here DO shoot 80 grains, and 90 Grains of FFFg powder in their guns, because those are the powder charges that shoot the most accurately IN THEIR GUNS.
You didn't ask for the BEST load using FFFg in our guns. You only asked if 70 grains of FFFg would be adequate to kill deer in your .54 out to 75 yards. We could only assume you had a reason relating to the gun, or how you shoot, that caused you to be shooting that particular charge of powder. A lot of deer have been killed with .54 PRB in front of a lot less powder.
Accuracy is the key to a successful hunt. That RB weight a bit more than 1/2 oz. and puts a huge hole into a deer. The pure lead ball will expand to make that an even bigger hole going through the vital organs, if YOU place the ball in the right place on the animal. And, that heavy ball will completely penetrate the deer on a broadside shot, causing massive hemorrhaging inside, and out of the deer. If you take a shot other than a broadside shot, the ball will travel a long distance through the deer's body, and deliver a mortal wound. Give the animal time to sicken, and lie down, and bleed to death quietly, before chasing after it. It won't go far if you don't keep pushing it.
Have a good hunt. :thumbsup: