gotcha. i think i'll go ahead and try it out with the cbc/f2 and dextrose. did you just use a regular priming calculator like for bottling?
I calculate it based on sugar weight by volume. You get about .51 volumes of Co2 per additional gravity point. I can't find my reference to that off the top of me head, but it was something from The Beerery/Low Oxygen Brewing website.
There are two tricks. One is figuring out how much Co2 is remaining in solution after fermentation/dry hopping. If dry hopping post soft crashing, I usually assume zero Co2 left, because dry hops dislodge a lot of Co2 from solution due to the additional nucleation points.
The second is figuring out how much additional attenuation you will get from hop creep. This is very hard to predict and varies based on hop variety/quantity/dry hop length/hop freshness/etc...
Assuming zero residual Co2 from fermentation, that means you need 5 gravity points to get to 2.55 vol Co2. You get some unknown amount of additional fermentation from hop creep (if dry hopping post soft crash). I assume 2 points, but it usually ends up being more than that. I usually shoot to add enough sugar to raise the gravity by 3 points. That equates to about 3/4lb of dextrose per 12 gallon batch. If I overshoot, I just serve the beer based using head pressure and no external Co2 until the carb level comes into line.