Don,
If the switch is in the RPM/Tach position, the top black scale looks like it reads 0-24. This looks like it would be the 4 cyl scale in RPM x 100. Therefore, it would be reading 0-2400 rpm on a 4 cylinder motor. IIRC you can just take this reading and multiply by 4 to get your RPM. Therefore your "1" would be 400 RPM and your "8" would be 3200 RPM.