Hmm it's fair enough. There's a Dark Heresy to Rogue Trader psychic powers conversion sheet that suggests the maximum overbleed is the psykers Will bonus times the Psy Rating they used for the power - what about something like that? So in this case the maximum overbleed would be 20? (5 will bonus times 4 psy rating)
I think it's a fan conversion sheet (though it looks professionally done), so it's a homebrew rule, but would that be a good limit?