there is also other simple rules...

the actual rule is to set shutter speed to whatever iso you are using and f16 for your aperture.

so iso200... f16 1/200 iso200

also other helps tips
Conditions: Sunny days
Av: f/16
Example of shadows: Dark with sharp edges

C:Slightly overcast
Av: Add 1 stop to f/16: f/11
S:Bright with fuzzy edges

C:Overcast
Av: Add 2 stops to f/16: f/8
S: Barely visible

C:Heavy overcast
Av: Add 3 stops to f/16: f/5.6
S: Absent

C:Open shade
Av: Add 4 stops to f/16: f/4
S: Absent

C:Backlighting
Av: Add 1 stop to f/16: f/11
S: depends


hope this helps.