I'm trying to improve my knowledge of surfboard, and there is a lot to learn. 

On my first surfboard, airush choptop (active) which is marketed as a somewhat flexible board, but seems pretty stiff to me. So I started reading about various constructions, stringers, materials....but i still don't have a good idea about what would cause certain flex properties and what flex would be considered high and low. 

Anyone got a good source?

don't worry about it. you will have to ride a lot of boards before that ever makes a difference. I'd say the only boards that are really flexy will buckle...except the ones with TT construction.