Recommended load for a Zoli .58 boarding gun?

[Jon80]

I've had this gun for a couple of years, haven't fired it yet. It's a hand cannon, what's the best load of powder for it, 40 grains?

[arcticap]

Pedersoli recommends loading between 25 - 40 grains of powder with a patched round ball in their Harper's Ferry .58 flintlock pistol.