herrdoktor:
Is your cartridge the straight case or the bottle-neck .500/.450 case? I load for a variation of the latter case, using 72gr Varget behind either a 300gr thin-jacket Taipan bullet, or the 350gr Woodleigh JSP. A couple of saddle-felt wads, or a hefty tuft of dacron, is needed to take up the air-space under the bullet. Read about my rifle and loads
HERE if interested.
Unfortunately I have no idea of powders available to you in Spain.
If your case is the straight-tapered .450 x 3 1/4, Graeme Wright suggests 70gr Varget, with wads, under a 350gr bullet for 1970 fps; or 55gr H4198 with dacron for 1950 fps.
Be prepared to go up or down a couple of grains for correct muzzle velocity to achieve barrel regulation.
Given the light weight of the Francotte, I'd be looking at the lighter bullet weight if possible. Graeme also lists a load of 97gr H4831 under a 300gr bullet for 1450 fps, and something in that range might be more comfortable to shoot, especially at the range.
Keep trying to find a copy of Graeme's book! It's pure gold!
Hope this helps.