What will your FG be? Well, as with all things that deal with yeast, the answer is "it depends."
There are so many factors that determine the attenuation of a certain beer that I don't think you could ever truly predict. Sure, we can make an educated guess and come pretty close... sometimes you'll even guess the right number, but I think this is just hind sight bias.
The best you can do is to use david42's formula. First, do some research on the yeast strain you're using. I use 1968 in most of my ales, and its attenuation range is 63-70% (I think). So pick the mid point of 67% (I've come to know the yeast and my system so that I usually achieve 69%).
So you want to brew a strong brown ale with this yeast, but you want it to have a drier finish. You decide on an OG of 1.070, and you decide that 10% of your fermentables will come from treacle (let's just assume treacle is 100% fermentable). WITHOUT the treacle, your OG would be 1.063. If you multiply 1.063 by 67%, you get an estimated FG of 1.021. But as I mentioned in my post above, adding sugar to this beer will increase the OG without changing the FG. So your apparent attenuation will be 69%.
I think this method is good enough for predicting your FG within say 3 points, but I wouldn't be surprised to be proven wrong. More importantly, you have a better understanding of what's going on in your beer. For example, I use 1968 because I love its malty profile and I usually like the residual sweetness it can leave, but in the case of bigger beers such as the one above, I will frequently add 5-10% sugar just for a little extra dryness.
Last thing I wanted to mention that just reinforces the "it depends" argument: I have a spreadsheet where I keep track of all my batches' attenuation numbers by yeast strain. I enter the percentage specialty grains, percentage sugar, mash temps, and then the OG, FG, and attenuation are all charted. I haven't been doing it too long, but already I see very little trending. The only things I've really learned are common sense anyway: darker malts and higher mash temps generally lead to lower attenuation, but even that is not a hard and fast rule.