The official way to calculate the key buffer usage as documented in the MySQL Reference manual:
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.1 - ((Key_blocks_unused * key_cache_block_size) / key_buffer_size)
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.
If the the key buffer is full:
key_buffer_coefficient = key_cache_block_size/(key_buffer_size/Key_blocks_used)
If the the key buffer is empty:
key_buffer_coefficient = key_cache_block_size/(key_buffer_size/Key_blocks_unused)
The 'fixed' formula:
1 - ((Key_blocks_unused * key_cache_block_size) / (key_buffer_size * key_buffer_coefficient))
The result is shown below:
The overhead in the key buffer will be allocated on startup, so the larger overhead for a larger key buffer will result in more memory usage, even if MyISAM is only used for the internal mysql tables.