The Sharps breech is somewhat counterintuitive when it comes to muzzle loader protocol.
In muzzloaders, you never leave an airspace between the powder and the ball.
The Sharps breech block has this huge empty chamber behind the gascheck plate.
Think of the breech block and gas check plate assembly as a steam valve.
The exploding charge "pops" the GC plate forward to seal the breech face upon firing.
That being said...
If you were to loose load, drop a lubed bullet or ball into the chamber. It will (should) stop at the rifling, so no worries.
Then you could fill the chamber flush with powder (this might total 70 to 90 grains of 2f).
Close the breech, cap and shoot.
Using a paper cartridge accomplishes the same task, only tidier.
IF the gun does not fire-
Wait, re-cap and try again.
Avoid opening the breech unless necessary. Any loose powder that might have entered the breech cavity (normal) could change how you perceive the charge volume, so do not add anything.
Any oil in the firing hole or nipple will put the primer spark out. So be clean and dry before you start.
If you need to unload without firing, de-prime, open the breech and dump it out, then use a cleaning rod or ramrod to push the bullet out.
Then drop the breech and inspect and clean as necessary.