Not so much a simple rule.
Some things to consider are overall rifle weight, barrel length and weight, chamber length (careful here, as some get lengthened over the generations), class of action (and breech depth), and of course condition.

The makers would at times (some more than others) engrave the charge prescription on the action, barrels, or buttplate. There was no requirement at the proof house for a service charge prescription on the black powder rifles.

We know what standard weights were from makers' records and such text's as Greener's Gun.

By appearance alone, I'd say start with the lighter end of the 10bore charge spectrum and go from there.
A 10bore rifle could possibly have been built for as big as 10 Dram loads.
I wouldn't start anywhere near 10 Drams though.

Cheers
Tinker