One more question what if I messed up and added the FWH along with 60min hops? I'm guessing my beer would be more bitter than the recipe call for. Right?
Your tongue will have to be the judge of that.
Some folks suggest you'll get the bitterness of a 20 minute addition from FWH, and to not adjust your bittering addition.
Others suggest that you'll get full bitterness from the hops, but it'll be "smoother" so it won't be as prominent.
Others suggest that you'll actually get more bitterness from them.
Others suggest that you'll actually get more bitterness from them, but since it's smoother it will seem like less.
And, yet, others suggest it makes no difference at all.
FWHs are not thoroughly understood, scientifically. Therefore we are left to our own perceptions to judge how they are utilized in our boil kettles.
I, personally, find them to be a regular old full boil bittering addition from the bittering perspective. I do not grasp the "smoother" character; in fact, I sometimes find their bitterness to be a bit more grabby. To me, the grabbiness works in some beers (pale ales, ipas, etc) but not so well in others (cream ales, Scottish ales, etc). Additionally, I _may_ perceive some additional hop character retained from FWHs, which again works well for the same beers and not so well for the others. Then again, this might be a fulfilling prophecy on my part, and hard to test since the beers I use FWHs are already very hop-present. So, for me, I implement FWH using my normal bittering addition on specific beers styles only.