I found that Gordon's recomendations are in line with my observed results. When I first got my zone VI pentax meter, I bought some 40.5 mm filters to mount on the spotmeter and metering through the filter, expecting, as per Picker, that my meter readings would be correct, using this procedure.

Empirically, I found my pictures using red, orange and yellow-green filter were under exposed by 2, 1 and 1.5 stops respectively. yellow meters accurately.

also be aware that this meter will underexpose by 1 to 2 stops when metering in tungsten light.

Take care,