I was able to find my paper with the settings that I needed.
Oehler says a minimum spacing of one foot is allowed but a minimum spacing of two feet is recommended. Spacing of 4 ft for high velocity rifles is best.
--one foot settings
sw1 on--sw2 off --sw3 off--sw4 off--sw5 off
2 foot spacing
sw1 off--sw2 on--sw3 off--sw 4 off--sw5 off
3 foot setting
sw1 on--sw 2 on--sw 3 off--sw 4 off--sw 5 off
4 foot setting
sw 1 off--sw 2 off--sw 3 on--sw 4 off--sw 5 off
s1 on--sw2 off--sw3 on--sw4 off--sw5 off
6 ft spacing
sw1 off--sw2 on--sw3 on--sw4 off--sw5 off
There are settings from 1 to 15 feet listed. Turning switch 5 ON multiplies the screen spacing setting by 10, or causes the displayed velocity to be multiplied by 10. Switch 6 controls the edit mode. If switch 6 is ON the edit mode or replay is available. The unit will store up to 20 shots and be replayed and individual shots can be omitted. Switch 6 OFF and you can fire and summerize up to 255 shots. The unit is shipped with sw6 OFF.
Switch 7 is normally ON if you have a printer. No printer, leave 7 off.
Don't know that you can set your screens at 18" and get an accurate reading. I'd use 2 feet if you could. This info is from Oehlers manual 1991 version and my 1991 35P has dip switches in the battery compartment next to the battery. They may have changed the unit over the years to another method of calibrating your screen spacing??