Right, one just estimates. All I have planted are flavor and aroma hops, unless you count Mt. Hood. I'll only used homegrown for bittering in ales that are on the low side anyway.
If you figure 12% for Columbus and shoot for 80 IBU, a 1% error gives you a range of 74-86 IBU.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk