I found out early in the game to use internet plans as a guide line, and then make all of your own measurements. For spacing the distance between the top and the runner I just turn the rack upside down and use the tub to make my mark. I have no idea what the actual measurement is.

If I had some help I would do a DIY video on building both the large and medium mixing tub racks. I just don't want to stop to video or take pictures. Its VERY time consuming to do for free when you're working alone.