A few thoughts:
While you can come up with a ballpark estimate of FG in advance, you won't know the actual number until the beer is finished fermenting.
Honey is pretty much 100% fermentable. That means that that it will boost the OG, but shouldn't affect the FG. To come up with a better estimate of ABV, when I use 100% fermentable sugars I use a calculator to estimate the FG without the sugar/honey/molasses/etc., then I calculate the OG with the sugars included. I use those two values (OG with sugar, FG without) to estimate an ABV.
As far as how much honey you can use, you can go as high as you want. The more you use, the more it will taste like a braggot (a mead made with malt sugar) rather than a beer. For a typical honey ale, especially a low-gravity one like your recipe, I would start with 1 to 1.5 pounds of honey for a 5 gallon batch as a good starting point. That generally gets you to a point where it still tastes like beer, but the honey does come through as well.