You may be getting too much sky in the meter reading. To avoid this while testing, take it indoors where the lighting is even and see what it reads against an evenly lit wall compared to your cameras. Alternately, use it outside in incident mode and see what you get then. I use a Soligor spot meter for general work, but have a 308 for flash use and they seem to agree fairly closely (about 1/3 of a stop IIRC).
Good luck! Bob.