I'm about to do the same thing to a split batch of saison.
It says on the can that it ranges from 1.040-1.055 IIRC.
My recipe is 11 gallons, and I'm adding the Rasberries to only one carbouy. So in a few months I can tell you how much gravity the one 49 oz. can added to my saison compared to the saison without it...
If anyone has a more theoritical way of calculating this or personal experience, I'd be interested in hearing it.