Regarding the difference between the grey card reading and the incident reading, it may in fact be the case that you need to incorporate a correction factor of .3 stops. Check the manual for the meter or the spot attachment.

There are also differences of opinion as to how to read a grey card. Some read it straight on and some read it at an angle between the lens axis and the main light source. See if reading the card in a different manner gives you a result equivalent to the incident reading (try flat, 30 deg., 45 deg., and 60 deg.).

With my Flashmeter III, I need to reduce exposure by 1.2 stops with the 5-deg. finder according to the manual (and my own experience), so I just add that factor along with filter factor and bellows factor to the ISO setting when I use the meter with the finder, and that way I don't leave anything out and can just take a straight reading.