I think you'll hit right about the ABV numbers you want.
If you aren't happy with that though, I would still leave it alone personally. Taste the wort at bottling and if it doesn't have the body and sweetness you want adjust with a little maltodextrin in with your priming sugar (maltodextrine is only 5% fermentable).
That'll give you a little more body and sweetness to stand up to your bitter brew if it is a little too fierce
Trying to squeeze a little more alcohol out of a wort that is already boiled and fermenting has always struck me as a risky proposition. Try to identify why you didn't get the OG you were shooting for and adjust in the future. And learn how to read that hydrometer and adjust for temp, it's the second most important skill for a brewer!