The only way to know what the maximum safe load for a particular gun is is to load it with gradually heavier charges until the barrel explodes when fired, and then back off about 10%. ::
Seriously, with that barrel - assuming it is in good repair and maintained properly (i.e. the breech pulg isn't a rusty, corroded mess, it hasn't been fired with airspace between the ball and the charge in the past, the sight dovetail and staple mounting dovetails weren't milled too deep, etc.) - you could put 90 gr of FFFg powder in repeatedly, but why risk it?. It would certainly not be your most accurate load. I would work towards something in the 65 to 70 gr range (start at your 55 gr and add 5gr at a time).
Here's a response from Dixie Gun Works to the general question.
What is the maximum load of black powder for my muzzleloader?
The short answer is to adhere to the manufacturer's recommendation for maximum load for your particular muzzleloader. However, this information is not always readily available. From a practical standpoint this becomes a moot question. Increasing the powder charge to near maximum levels will lead to diminishing returns on the efficiency of the load, as you will experience significant increases in pressure with only marginal gains in velocity with an adverse effect on accuracy. As a general rule, a good target load can be established by starting with a grain weight charge equal to rifle's caliber ( eg 50 grains of FFG for a 50 caliber rifle). To find an efficient hunting charge, increase the charge in 5 grain increments to the point where accuracy is acceptable for your hunting situation. Generally, this should be limited to 30 percent greater than the most efficient target load.
Shooting the flintlock upside down, beside being hard to sight the target, just shows that the frizzen holds the powder in the pan until the sparks strike the charge. It might actually be FASTER than right side up, because explosions will still travel up, away from the earth, and the bulk of the powder is out of the way of the flash. A flintlock pan charge should not fire like a fuse. It should flash instantly into the vent. 3 or 4 grains of powder is a good prime and more can slow the ignition down (and hold the frizzen open a crack so moisture can find it's way in easier).