Notes on aperture readings
Aperture readings can be made either using incident or reflected lights. The values you'll input should all be from the same type of reading and measured at the same point, otherwise it won't work properly.
You'll also soon discover that adding two times the same value will result in a slightly different value than what you are most likely to know. This is because we normally use the simplified values instead of the exact values. That's why you'll find two lights at f/5.6 are f/7.92 instead of the usual f/8. In reality it won't matter much anyway.
Do you want more calculators?
Here is a link to a list of photo calculators.