The image below are two sides of a card (credit card size). Feel free to print it and use it as long as you don't sell it. To squeeze things into a small space, the instructions on the card are not detailed enough. Let me know if you have questions about how to use it. It's really very simple.
Please let me know if you like it (email@example.com). If there is enough interest, I'll find a place to make a few hundreds of plastic cards (like a credit card). I guess I may have to charge a couple of dollars per card to cover the cost.
Note: To use it for focal length greater than 100, devide the focal length by 10 to find a value in the table and devide the value by 100.
Here is the PDF file. Feel free to print it and use it as long as my copyright notice is not left out and you don't sell it. Have fun!