As a general rule that I use. And I don't think this is the correct way to do it, but... If the 727-200 is struggling to maintain a climb of 200-300 fpm AND 260 KIAS, I'm probably flying too high for the weight. I'll descend about 1000-2000 feet if that starts happening. That's when it "feels" like it's just too heavy to go any higher.

I'm going by the "feel", if you can call it that.

Mark