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Dean --
I specified spot measurement originally because I was aiming this system at zone system users and I assumed that few of them used or owned incident meters. In fact, though, the method of test metering is immaterial because you're simply establishing a personal standard setup. So, whether you use spot or incident metering, adjust the brightness empirically to place the speed point of an honestly-rated film/dev combo at 2.4, then document your setup (enlarger height, aperture, time, EV, ISO). It's convenient to adjust things to get a simple EV number (e.g. 4.0 rather than 4.2 or 3.7 etc.) and be sure your setup provides enough latitude to handle films as slow as ISO 25 or 50.
You're right, the spot readings of white are higher because the incident meter is reading "gray." Either method is OK but if you want to make them more similar just remove the incident meter's translucent dome.
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