Gents: My .450-400 3" Harrison and Hussey ble was originally regulated for the 55-grain tropical charge. I have been shooting 80 grains of IMR 4831 for 15 years and it groups about 2 inches at 50 yards. Due to the bulk of the slow powder I do not use any fillers but always shoot large rifle magnum primers. I've seen like rifles regulate between 78 and 83 grains of the same powder.
cheers,
Cal