it's a .54 so I'd say maybe 35 grains of GOEX FFF)?
35 grains is way too light for a .54
Anyway, you have had some good advise already here, but I'll add my own 2 cents;
There are commercially available powder measures. Some are fixed at a certain amount, and others are adjustable. These are usually made of brass. Most any place that sells black powder gear will have one or more.
You will need to shoot a lot in order to find out what your particular rifle likes. For a .54 you could use either 2F or 3F real black powder. With that, I would start with 60 grains. Fire 3 -5 shots for a group. Try 50 yards, but if your groups are too large or too far off target at that range, close in to 25 yards. Use a solid bench rest. However, don't worry about being exactly on target at this stage. Just try to get all your shots grouped as tightly together as possible. Go up to 65 or 70 grains of powder and fire another group of 3-5. Keep going in these increments up to about 100 or 110 grains. You shouldn't need more than that, and those are pretty stout loads! But, there are guys who use 120 grains, or even 140 in their .54 rifles. If you like getting kicked by a mule, then go for it. Decide which load gave you the best group. There you are.
Often a rifle will have two different loads that it likes; a target load and a hunting load. Target loads are usually lighter. Hunting loads are more potent, but often not quite as accurate.
Once you've decided what load is best, stick with it while you adjust your rifle sights to get on target.
Also, many of us like to make our own custom powder measures. They are easy to make. They can be made from a stick, a wooden dowel, river cane, horn, bone, whatever. The one I currently use I made from the femur bone of a deer. Just drill a hole down through the length of whatever material you choose. Personally, I like a 3/8" diameter hole, but it doesn't matter. Pour in your preferred powder charge and make a mark slightly above the level of the powder. Pour out the powder (save it), and cut off the horn or whatever at the mark. Whalla! Drill a small hole crosswise through the other end of the material for tying on a thong. I like to varnish the outside of my homemade powder measures to help waterproof them. They could be painted, too. Use your imagination.