D-180 will add about 32 points per pound per gallon. Let's say you'll have 5 gallons of wort post boil, including the two pounds of D-180. The two pounds of D-180 will be providing 64 "points." Those points will be distributed throughout 5 gallons of wort, so...
64 / 5 = 12.8 points. (Round to 13 gravity points.) So, if your wort would have had a post boil OG of 1.085, the OG including the syrup would be about 1.085 + .013 = 1.098 (at 5 gallons).
But there's a catch. The 2 lbs of candi syrup will also add about 0.1625 gallons of volume, so if you just add it to the same wort that would have hit 5 gallons post boil without the syrup, you'll have 5.1625 gallons. And the gravity would be a little lower:
98 x (5 / 5.1625) = ~95 , so 1.095
If you wanted to hit 1.098 @ 5 gallons (and not 1.095 @ 5.1625 gallons), you'd need to boil longer or use less total water.
The rigorous way to calculate the effect of fermentables added to the boil is to do a mass balance on the water and sugar (actually extract or total dissolved solids.) In this case you don't need to know the volume of items added by weight.
In the case of D-180 you know the total weight and pts/lb (32.) You can convert pts/lb to % sugar by dividing by 46.173 (which is the pts/lb of pure sucrose.) So, D-180 is 32 / 46.173 = 0.693 or 69.3 wt% sugar. Therefore 1.0 lb of D-180 is 0.693 lb of sugar and 0.307 lb of water.
My spreadsheet calculates the total weight of the wort in the BK, the weight of extract in the BK, and the weight of water in the BK after mash and lauter. Weight of wort = weight of extract + weight of water.
The original Plato of the wort in the BK is: 100 * Weight of Extract in BK / Weight of Wort in BK. Which is the same a weight percent of extract in the BK.
To calculate the Plato after an addition to the BK you use the following formula:
New Plato = 100 * (Original Extract Wt + Added Extract Wt) / (Original Wort Wt + Addition Wt)
case* my spreadsheet gives original extract wt in BK as 10.392 lb, and original wort wt in BK as 63.548 lb, and the original Plato as 16.35 (or 1.0670 SG.) So, the new Plato will be:
New Plato = 100 * (10.392 + 2 * 0.693) / (63.548 + 2 * 1) = 100 * 11.778 / 65.548 = 17.97
You can use the following formula to convert Plato to SG:
SG = 1 + (Plato / (258.6 - ((Plato / 258.2) * 227.1)))
So, the pre-boil SG, after addition, would be 1.0740.
After boiling off 1.65 gal or 1.65 gal * 8.3304 lb/gal = 13.745 lb of water, the OG without the D-180 addition would be 100 * 10.392 / (63.548 - 13.745) = 20.87 or 1.0869 SG. After boiling off the same 1.65 gal the OG with the D-180 addition would be 100 * 11.778 / (65.548 - 13.745) = 22.74 or 1.0953 SG.
* My numbers will be a little different as I don't know exactly what @SRJHops
's inputs to my spreadsheet were, but I should be pretty close.