Where can I find a list of rules of thumb formulas for navigation?
There is no short list but almost all the navigation information you could ever want can be found in this handbook published by the FAA. If you are worried about your nav computer failing you can always get an E6B which is about as close to failure proof as you can get. An E6B will expedite most most nav related calculations in the air (like headwind component). My E6B has some common nav calculations written on the back of it actually.