Mike,
Thanks for the comments.
I've never had a lot of luck in chronographing my 12 bore loads, I have a CED Millenium II Chronograph, and for some reason, it doesn't want to read these loads, I use a plastic wad plus a card filler wad under my projectiles, and I think they play havock with the start/stop screens, don't think it knows what to do with the wad material.
However, with one of the loads I use, on about 5 separate occasions I have had 1 or 2 of them read, and the rolling average is around 1415 fps, that's with a 680 grain CBE Hollow point "Paradox" style bullet.
But I wouldn't take that as gosphel though.
Loads? The best thing I could do would be to direct you to Graeme Wright's "Shooting the British Double Rifle" book, that's where I got my starting loads from.

A hint - don't overlook Herco as a really good powder for these loads.

Regards,
David.