I read an article a while ago about a cruiser who was "doing the ditch" with a mast that was near the maximum for the ICW (what's that, about 60?). He would hunt for straight sticks about 3 feet long, that had a branch at about 45 degrees. He'd attach that to his masthead with the straight part pointing forward, and the 45 degree branch pointing forward and up. The 45 was the height of his antennas, the straight was the height of his spar. Then he'd approach bridges VERY SLOWLY, and watch to see if the top branch touched the bridge, at which point he would back out.
Sounded insane to me, but apparently he went through the entire waterway this way, periodically replacing sticks when he'd break them on a bridge.
I'll tell you from my experience, that 10 feet of clearance over your 50 foot mast looks like certain doom when you're standing in the cockpit looking up at it. From a shallower angle of view I'm sure it would look fine, but from the boat, it really looks like you're going to hit, so it's important to use chart datums, and trust them, because if you're going by eye, you'll never have the nerve to do it.