Since the tubs are never the same, I would measure each one, use the tallest one and add 1/8" to it. I cut a jig out of a scrap block so all of my shelves are the same measurement. Cut two strips of scrap shelving the height you want and place them for your spacers. It will save you time, not to mention it's always the same. When using spacers and tubs, tubs can sometimes flex causing inconsistencies. If I get time, I'll take a pic of what I'm trying to say. lol.