Hi bikewise55 , and welcome,
Not sure if this will help but it is the tyre sizing chart that came with my monitors, which whilst the cheapest on the planet read almost identically on a small and full sized bike. As I have two identical monitors.
As others have said just get the tyre size off the tyre and match it up.
Or do a roll out on the back wheel, and do a measurement on some level ground and then compare it to the chart. Just start from the valve in a vertical position and finish the same.