Thank you CL!
I made a strip of wood to space the lower runner off the bottom and another strip to space the other two from that lower one after I installed it. I hate installing runners by calculating so I triple checked everything then used double side tape to test fit it all first. Since the drawers were not yet assembled, the tape was strong enough that I could place the drawer sides over the runners and measure the gaps.
I suppose if I did this regularly, I could trust my initial guesses.