There are really no standard conversions that I've ever seen and it's important to note that powders' behavior is not linear. They vary from cartridge to cartridge and I have never seen an algorithm that could be safely or effectively used. I just looked at my RCBS Load software and couldn't find anything similar, either.
I have taken a powder that has similar load characteristics to the original and worked loads up from a baseline (usually about 20% or more reduced), by ½ gr increments. I’ll use my chrono and watch for signs of overpressure. But that is not completely reliable if you don’t have a strain gauge or other way of accurately measuring chamber pressure (which I do not).
ADDENDUM: I just did a search for "Herters .401 Powermag" on Yahoo and found some interesting info, as well as some references to discussions on other forums. Might be of some help. ~GR