I have a scale that measures it.
It's not super accurate, but it at least gives me a relative measurement (I know where I was relative to the day before, and the week before).
And it's at least ballpark-able. When I see 26.8%, I know that's too damn high. I may actually be 25% or 29%, but the margin isn't the big deal right now.
Just depends on who you are and what you're trying to do!