LOL... I figure that the term 'middle rack' was because perhaps a lot of ovens in the early days didn't have equally space racks or as many slots as some oven do now.
says, just eyeball it or measure. I would say it also depends on what you are cooking. If it is in a deep/high pan, such as a loaf pan, I would take the lower middle. If it is a shallow pan, maybe the upper.
I've had the same situation but in the end I've found it doesn't really matter. You'll have to figure out your own oven idiosyncrasies.
Just keep an eye on it, especially towards the end, to ensure it is fully cooked for whatever it is.
Another suggestion might be to use an oven shelf thermometer. Start placing it on the two racks at different times (or get two, 'bout $4 ea) and see which one comes closest to the desired temp. Of course, I should think, is that the temps will change depending on what you are cooking.
I believe that the idea of the "middle rack" is that there is equal temp surrounding/blanketing the object being baked/roasted. IMHO