With the 3/8 diameter by 1/2 inch deep chamber in your fairly thick walled 50 cal cannon I don't think it would actually be possible to overcharge it with any amount of any type of powder that will fit in the chamber.
What you have is pretty much in the same class as a thick walled black powder pistol, and there is a sub-chamber to reduce the chamber size even further.
I would base your charge on recoil more than anything else. If you are shooting 50 caliber balls the recoil will move the gun back quite a bit. You want to launch the ball, not the gun.
You can certainly start with 2F, but 3F is not out of the question either if 2F won't get the velocity you want.
You can try patching the ball as well, although loading a patched ball in a smoothbore is harder than in a rifled barrel. Try a looser fit patch/ ball combo first. 490 molds are common. A 490 mold with a .010 patch will fit tight. If you can find a .480-485 mold that would maybe be a better choice. I don't know where you can get one... Maybe consider a thing as small as a .457 ball with a thick patch.
For a frame of reference, see
somehttp://www.traditionsfirearms.com/files/Cannon_manual.pdfremember that their cannons are rifled, which gives the patch someplace to go. The recommend 20 grains 2F as a max charge for their 50 cal, although they do mention without recommendation as to quantity that 3F can be used in their smaller guns (50 cal is their smallest gun).
Lacking a subchamber, their guns have much thinner walls than yours.
Also, look here :
http://www.traditionsfirearms.com/eshop/products/TRADITIONS%20SIDELOCK%20BOOK1.pdffor BP Pistol loading data. If you call your gun a .32 roughly the sub-chamber size) the recommend 20 grains 3F max.
My CVA 45 caliber cannon recoils so much with the max charge they recommended (I forget offhand what that was) that I cut way back to keep the gun from leaving my yard.
Start low, and base things on maintaining reasonable recoil mostly. Keep the charge under what the above references (for thinner walled guns actually) recommend and you will be way safe.