Calculating Gravity with Addition

I recently made a cider. I took the initial gravity reading when I first put it in the fermentor, but when moving it to the secondary, I added about a pound and a half of raisins and let it ferment for another couple of weeks.

My question is this: When adding something in the secondary fermentor, how do I calculate it's impact on my final gravity reading?

I guess that it's this way, but I'm probably wrong and that's why I'm asking...
(Primary OG - Primary FG) + (Secondary OG - Secondary FG) = Total FG

Is this correct, or completely wrong?

Sure - that's a reasonable way to estimate it - I assume by Total FG you mean effective delta FG. It's not exact, but close enough. My question is this: could you really measure the effect of the raisins in the secondary OG? I'd have thought the grape sugar would dissolve over time.

Also a good question, but one I am not nearly experienced enough to answer. Lets say I was adding coffee into the secondary fermentor of an oatmeal stout. Would I use that formula to calculate an accurate final gravity?

No - coffee has no fermentables, and the tiny amount of dissolved solids will give it an OG negligibly different from water. So the simplest thing to do is just derate your primary OG by the amount that you're changing the volume by adding the coffee. For example, if you increase your volume by 10% by adding the coffee, and your primary OG was 1.050, your effective OG is now around 1.045.

Ah, good info! Thanks!

Another question, though, because I really am just looking to increase my knowledge of this whole process. Say, hypothetically, I added some amount (let's say, half a pound) of pure sugar. Yes I know the beer would taste awful, but how would I go about calculating the gravity of that?

I guess I'm just trying to understand how the addition of anything effects gravity readings.

As long as you know your volume, it's pretty straightforward. Sugar has an extract of 46 p/p/g. The unit is actually point-gallons/pound. So 1/2 lb of sugar in 5 gallons is 46*0.5/5.0 = 4.6 points (round it to 5). In other words, that OG that was now down to 1.045 because you added the coffee (and brought your volume up to 5 gallons in the process) is back up to 1.050.