Marko, you might benefit by learning the SLAT rule. EV = S + L = A + T (where EV = the Exposure Value: S = your film's speed index number (ISO): L = the Additive Light Value , numeric value of the light reflecting from your subject: A = your aperture setting: T = your shuTter speed setting)
EV = A + T
Shutter speeds and apertures are given numeric values that combine to give you an exposure value (EV).
Shutter speeds
1 second = 0, 1/2 sec = 1, 1/4 = 2, 1/8 = 3, 1/15 = 4, 1/30 = 5, 1/60 = 6, 1/125 = 7, 1/250 = 8, 1/500 = 9, 1/1000 = 10.
Apertures
f/1 = 0, f/1.4 = 1, f/2 - 2, f/2.8 = 3, f/4 = 4, f/5.6 = 5, f/8 = 6, f/11 = 7, f/16 = 8, f/22 = 9, f/32 = 10.
Say you meter a scene and read an exposure value of 13, then any compination of settings that total 13 will give an equal and average exposure to your scene, ie. 1/60(6) at f/11(7). 6+7=13. And any other combination as well.
EV = S + L
For Additive light values and film speed indeces, the same roughly applies.
Film speed
ISO3 - 0, ISO6 = 1, ISO12 = 2, ISO25 = 3, ISO 50 = 4, ISO100 = 5, ISO200 = 6, ISO400 = 7, ISO800 = 8, ISO1600 = 9, ISO3200 = 10
Additive light values (in candles per square foot (c/ft 2)
6 c/ft2 = 0, 12c/ft2 = 1, 25c/ft2 = 2 50c/ft2 = 3, 100c/ft2 = 4, 200c/ft2 = 5, 400c/ft2 = 6, 800c/ft2 = 7, 1600c/ft2 = 8, 3200c/ft2 = 9, 6400c/ft2 = 10.
There are tables of rough estimates of your light source depending on bright sunlight, open shade, night illumination, etc. If you know the approximate c/ft2 of your light and you can get your film's speed index then you can get the exposure value ie. the light of the full moon (roughly 250 c/ft2)(5), ISO1600(9) then your exposure value would be (5+9) 14. Then you choose a S/S-f/stop combination that equals an exposure value of EV14 (f/11 at 1/125 and so on).
S + L = A + T SLAT
The combination of shutter speed and aperture will be equal to the combination of the light reflecting from your subject and your film's speed rating. For rough figuring of exposure you now have two ways to go about it. 1.) EV ratings on your light meter and the S+L combination of light and film speed.
There is more information such as this at
www.fredparker.com. The Ultimate Exposure Calculator.
Hope this helps.