I use Unique in just about everything...from my little antique IverJohnson 32S&W all the way through the 45-70 and the 500S&W...probly one of my favorite powders.
An accidental double charge will really fill most cases so you can keep Unique much safer than the other fast burners like Titegroup. But the 460 is Big so keep an eye on things.
Since you are looking for a plinker here,,,I would start around 10 grains of Unique and see what my Chronograph tells me. If you don't have a Chronograph...You really need one to work loads from scratch because it will tell you things that can save your bacon.
It is bigger than a 454Casull so the 454 data will be a great start...
http://www.freedomarms.com/loadingdata.pdf