I would just stick with the 3/4 cup for five gallons and see how it goes. A bit of research revealed that you might need to wait about a month for it to carbonate. Sounds like a good experiment, I would be interested to see how it goes.
* Edit* I re-read your first thread and I guess that I forgot that we were not dealing with just Evaporated Cane Juice alone, you have it as a additive in cocoa powder, hmmm.......Nothing to do but either just try some out, or get ahold of the company and figure out the sugar %.
You could separate 1 gallon at bottling, try out 1.6 oz of the stuff (would equal 1 cup for 5 gallons) prime the rest with somewhere between .5 and .75 cups of regular dextrose and see what happens.