For MyISAM one of the most important variables is the Key
Buffer. The Key Buffer is sometimes called the Key Cache.
It's used as a buffer for the indices of MyISAM tables. There is
some overhead in the buffer depending on the configured key block
The official way to calculate the key buffer usage as documented in the MySQL Reference manual:
1 - ((Key_blocks_unused * key_cache_block_size) / key_buffer_size)
This will return the factor, so you have to multiply it
with 100 to get the percentage. The Key_blocks_unused is used
instead of the more obvious Key_blocks_used. This is due to the
fact that Key_blocks_used is the maximum number of key blocks
ever used. It will not return to 0 after a FLUSH TABLES.
This calculation does not take the overhead in account. The key buffer efficiency can be calculated if the key buffer is empty or (has been) completely full.