Thanks for your response. Yes, I've used the quilt itself to measure for borders (always in the middle) but then I don't usually measure the border to see how long it is, I just cut another the same length. So I often don't actually know the number it measures. And I don't usually measure middle and both sides. I'm having trouble with the borders on the quilt I'm currently making so I did take those three measurements today, both width and length.
On the floor is hard for me, too. For the record, I do use a tape measure...if not the border fabric....but I wondered if this is what others do, too. I hadn't thought of folding the quilt in half, dumb me!