I'd say that your projections for Burkhead's carries/targets percentages on 300 offensive snaps are pretty much spot on. However, I still disagree about his likely ST snaps.
If Burkhead sees 300 snaps (19/game) as a regular, skill-position contributor on offense, I'd say he's far more likely to see only about 80-100 ST snaps (5-7/game), with a maximum of about 150 (9/game), and almost certainly on blocking units only - no coverage/tackling units.
First of all, these ST projections for Burkhead would match league-wide SOP usage of offensive skill players on ST, and actually exceed what the Pats have typically done, since snap-tracking began. Secondly, unless there's an injury to one or more ST specialists, it really won't be necessary for Burkhead to do more than 5-7 ST snaps/game, and probably not even that much.
If Burkhead gets up to 200 or more ST snaps for the season, that would mean he'd participated in at least one coverage/tackling unit and likely mean that his primary usage had been on ST rather than on offense, where he'd be getting only "token" snaps (7 of fewer/game). In that case, the only way he ends up with 300 offensive snaps on top of those 200 ST snaps for the season is if one or more of the other RBs is injured for several consecutive games - during which period his ST snaps/game would drop dramatically (possibly to zero) as his offensive participation increased.
Again, if you look at the game-by-game snaps counts, this was Burkhead's usage pattern in Cinci. It's been the Pats usage pattern with Bolden. And it's the typical usage pattern with offensive skill players for all the other teams in the league.