I'm just curious for my sake: why add the candi sugar after the boil? Generally I add ingredients post-boil to preserve aromatic qualities and certain tastes that a boil won't be kind to.
He's not talking about immediately after the boil. He's talking about after a few days of fermentation.
The reason being that with a high gravity beer with a high percentage of sugar (like many Belgian strong ales) if all the simple sugar is there from the beginning the yeast will go after the simple sugars first, and end up not attenuating the malt sugars fully, leaving a beer nowhere near as dry as a Belgian beer should be. But if you add the sugars after fermentation has started, the yeast will go through the maltose before the sugar.
I usually wait until maybe 4 days into fermentation, ie when it's mostly done, but not quite at FG and the yeast are still active.
If you've got that hard candi sugar already, might as well use it. But if you haven't bought the sugar yet, just use corn sugar instead. It's essentially the same thing, just a lot cheaper and you'll get more bang for the buck. If you're going to use an actual candi SYRUP, that'd be a lot better. That's what the Belgian brewers use.
As far as how to calculate, take the ppg of the sugar you're using (I believe candi syrups are usually roughly 32 ppg, where the sugars are closer to 38 ppg) and multiply by the number of pounds. Then multiply the post-decimal original gravity reading of the batch, and multiply that by the volume of the batch. Then add those numbers together, and divide that by the total volume of both the original batch plus whatever volume you increased with the sugar addition.
For example, you're adding 2 lb of 38 ppg candi sugar into 5 gallons of 1.070 OG wort. 2*38 + 5*70 = 426. If after boiling those two pounds of candi sugar in some water, you're adding an additional .25 gallons of sugar/water solution into the 5 gallons of wort, then it's 426 / 5.25 = 81.14, or an adjusted OG of 1.081.
With sugars I boil them briefly in as little water as possible before I add them back into the batch. I'm partial to the syrups, since I think they add a lot of great flavor, and I figure since they were boiled during manufacture and packaged sealed, I just dump em right in without boiling (YMMV). In that case, I assume the volume increase is pretty much negligible, and I calculate as such.
Hope that helps.
