Calculating amount of time for brute forcing ciphertext depending on the size of the key
Your approach is too simple to give an accurate rating of the cryptographic systems. For any system that does not inherently suffer from short key lengths it is first and foremost its resistance to cryptanalysis that makes or breaks it.

The Vigenère cipher for instance is hundreds of years old and can easily be used with a very long key if so desired. That however does not prevent it from being a very poor algorithm by modern standards. It leaks so much information that if you know what to look for and have a decent length of message to work with you don't even need a computer to break it.

There can be no single way to compare wildly different cryptographic schemes, the very nature of their weaknesses differ. The best thing you can do is to list the requirements for breaking a given scheme, like:

Vigenère cipher:


A guess about the contents and words likely used in the message.
One or more messages using the same key, combined at least around double the length of the key.
A person who is reasonably good at performing basic algebra.
Some hours of that person's time.


Or:


One or more messages using the same key, combined at least around double the length of the key.
A person who understand and can use pattern frequency analysis of the language in which the message is composed.
Some days of that person's time.