I'm not too familiar with the candi-sugar. If it is a solid, I'd treat it like it was cane sugar. If it is liquid, treat it like it is honey, or other syrup.
In checking Palmer, I get about 4.75 ozs of cane sugar for a beer stored at 65 F, and 3 volumes.
You will only have 2 volumes if you use 2.5 ozs of sugar. I assume it is Belgian. They are usually carbed on the high side.