'76 Jon Timing Question

[Max Thrasher]

My manual states that to set the timing the engine needs to be running at 3,500 RPM's in either a test tank with test wheel or in the water with a prop, with timing light attached. Does anyone know if the same thing can be done at idle speed instead of 3,500 RPM's?
1976 Johnson 55hp. 55EL76

This cannot be done because the idle timing is fixed and the high speed timing can only be adjusted at 3,500 RPM's or greater. My neighbor just informed me about this.