I forgot to mention... If you test with Tungsten lighting, and your meter is overly sensitive to red wavelengths... While your film is "more or less" sensitive to near infrared... Then your "speed" will be lower than ISO [meter needle is higher than it should for the actinic light] and the Tungsten "speed" difference from Daylight may vary between different films of the same ISO rating. This is because meter spectral response may depart from film spectral response. The Zone VI modified spotmeters are intended to correlate well to Tri-X - so if you have one of those, then your tests may correlate better. But change film to a tabular grain type film and the correlation may not be the same. I've tested 400 TMAX and find it can take a very long exposure to a near infrared (visible to the eye) red LED with little fog.