The exact number isn't terribly important, but here are some things to consider.
-If your goal is strength because you want to be strong in a strength sport, those all have weight classes. Unless you plan to become a superheavyweight (in excess of 308lbs in powerlifting and 265 in strongman, possibly more depending on the federation), carrying a bunch of excess fat will work against you. Fat isn't muscle, and it's just going to move you up to a higher weight class, where you'll compete against dudes who AREN'T carrying a lot of extra fat. If you look at the 220s and below for powerlifting, the winner all tend to be jacked. Even the 242s aren't carrying a lot of fat. At 6' tall, you're probably going to be in those weight classes, unless you decide to use some chemical assistance.
-Going along with strength sports, if your goal is to do strongman, there is a moving component to it, and carrying a lot of extra non-muscle mass is just going to make you slower, so again, not good for you.
I've done the whole "super bulk" thing and got plenty fluffy, and it was pointless. You can only eat yourself so strong so quickly, and putting on a bunch of extra fat doesn't help. If I were to throw out a number, I'd say there's really no reason to go about 15% for a strength ATHLETE. If you just want to move heavy stuff though, with no real ambition of competing, be the bodyfat you want to be.