For me, 1.018 is usually my upper limit. I brew lighter beers though, usually. I would think that with a porter, FG might be a few points higher. I wouldn't worry too much. I brewed a beer with a lot of unfermentables in it a couple months ago and it finished at 1.020. It tasted like crap, but that was because of the recipe-- carbonation was right.
To be safe, you might want to bottle condition in a way that will not cause a huge mess in the case of a bottle explosion. I put mine in my bathroom for the first couple weeks, so on the off-chance that one does explode, I'm cleaning sticky off of lenolium instead of carpet.