Newbie type question for E-5

Hello,

I have an E-5 (various HG lenses) and starting to dive into how to be photographer after getting the gear (I jumped in too really deep with the gear, frankly too deep). After reading a bunch of books and practicing the relationship between ISO, shutter speed, and aperture (which I understand), I want to better understand lighting. Incandescent vs reflective, how to correctly meter different types of situations, etc. It has been a blast.

In studying I came across the sunny 16 rule. After practicing the sunny 16 I realized that I have to stop up to get the correct exposure. For instance, if shooting at ISO 200, f/8, and 1/800 the images were extremely underexposed. I stopped up to 1/400 and some images were appropriately exposed and some others a little overexposed. At 1/640 they looked a little better.

My question is this, does the sunny 16 rule only apply to traditional 35mm (FF) and does 4/3 have to stop up to compensate for the smaller sensor? Is there a hard and fast rule comparing FF to 4/3 regarding correct exposure? Am I missing something?

Any help would be appreciated. Thanks!
Matt