I would just use manufacturer's data for a bullet of similar weight, and start at the low end to be safe. I imagine frangible bullets can't be driven as fast, but I don't really have any experience with them. I have used the Barnes Varmint Grenades which are a copper jacket with a powdered metal core, so those may be similar. They seem to be most accurate at a couple grains less than max load, but YMMV. I've heard of guys driving them too fast and having them fly to pieces.
It would really suck to have one come apart in your suppressor....