Well, since you're adding the blackberries to secondary, your pitching rate is based on the OG of 1.078. Once that's done, there will be more yeast in the fermenter and you can rack onto your blackberries. Since the yeast multiply before fermentation, you should have plenty of yeast to work on the blackberries. The only issue could be the ability of the yeast strain to attenuate that much- from 1.078 to 1.022 is 70% attenuation and over 7% ABV. You may have a hard time pushing it any further, depending on the yeast strain you choose. Some strains might only get you 68% or so. Some may go closer to 80%, so if you want some fermentation of the blackberries you may want to chose a highly attenuative yeast strain. What strain are you planning on?
Beersmith is USELESS in predicting FG. That's up to you to predict.