Hmm I never used aperture that small. I'm still getting used to the aperture priority mode and haven't got to the manual mode yet ... The smallest aperture I've ever used is around f6.5. This could be the source of my problem ...
But I always take photos of flowers, small stuff, this and that. Could be why I constantly used large aperture to get the desired depth.
Hmm interesting. It's cloudy for a week and I need to wait for a sunny day to test your rule.

Thanks.