I am not any sort of authority on the the 11 mm German Service Revolver, nor am I versed in loading Pyrodex into metallic cartridges, but if you're brand new to reloading, my advice would be to learn the process first with a more modern weapon for several reasons.
The first is your personal safety, 130 year old weapons, were not built with the safety features that more modern firearms have, the steel was nowhere near as good as that used today and time is not friendly to metal, particularly springs, such as the ones that control timing and lockup on a revolver.
Safety aside, there is also a historical aspect to your revolver, if a major reloading mistake (read KABOOM) is made with a $200 Taurus .38, as long as no one's hurt, it's not a big deal, the weapon can easily be replaced. If a historical weapon like your Reichsrevolver is destroyed however, it's just gone and a little bit of history died with it.
Lastly, I think it would be be somewhat of a challenge to reload for your weapon and I've been reloading for almost 40 years. Black powder has all sorts of little quirks ( I've never used Pyrodex), and the bullet is an odd size, (.426 according to C. O. T. W.). There's no reliable data printed for the cartridge and to top things off, you'll have to use dies designed for the .44 Special and .44 Magnum and the only way that I can envision getting a proper roll crimp would be to mill about twenty thousandths off the bottom of your crimp die, although a Lee factory crimp die might work.
I'm not trying to discourage you from reloading, I just think it would be easier, safer and more rewarding to learn the basics on a more modern weapon that has proper cases, data and dies easily available. Once you have the basics mastered, you can move on to more challenging things, like reloading for your 11mm revolver, it'll still be there.
Swampman
PS, Did the Gunsmith actually test the weapon, or just look it over? If it was actually tested, ask him what loads he used and keep the info for when you begin loading.