The Original Rooster
Mayor of Spring Hill
Go with what @Railroader said and start at 50 grains to get it on paper and then experiment up to 90 grains if your gun is rated for 90. Certainly no need to go above that because that will provide all the velocity you need. Lots of deer have been killed with a .50 caliber ball with 50 grains of powder so if 50 grains did that with a round ball, you know that 70 with a 385 Great Plains or Maxi Ball will do the job.