AussieTim,
try Graeme Wright's Nitro for black load 440gn BP Woodleigh (Cat No8), 72gn AR2208 with a 20mm foam wad slightly compressed, Federal 215 primers - recoil will be 1/2 of the full nitro loads.
Very emphatic killer of pigs I can tell you!
Yes it means another powder but you can shoot more often.
Then if you go with Underlever's way - use as a BP rifle, you can cast your own projectiles like myself & get right into paper patching...now that's fun!
Ash's idea will be the lightest recoil using Trailboss - start at 70% & work up from there.
There could be a big change in point of impact, my .404 soft loads are 120mm or so higher at 50m.
Have fun deciding....try them all in fact, its a great calibre to play with.
Painful loading small calibres now-a-days after loading for a 500!
Cheers
93x64mm