What's the formula for calculating Key Efficiency, Key Buffer Used, and Query Cache Hitrate in MySql?
For your reference i am trying to explain some concepts also.

Key hit rate 

There are basically two forms of key hit rate

1.Key Read Efficiency


  Key Reads: The number of physical reads of a key block from disk.
  
  Key Read Request: The number of requests to read a key block from the
  cache.


Key Read Efficiency = [1 - (Key_reads/Key_read_requests)]*100



  Key Read Efficiency: The ratio of the number of physical reads of a
  key block from the cache to the number of requests to read a key block
  from the cache in percentage. The MySQL performance is good if the
  value of Key Read Efficiency is 90 percent and above. Increasing the
  size of the cache improves the value of Key Read Efficiency and hence
  an improved the performance.


2.Key Write Efficiency


  Key Writes: The number of physical writes of a key block to disk.
  
  Key Write Request: The number of requests to write a key block to the
  cache.


Key Write Efficiency=(Key_write/Key_write_requests)*100



  Key Write Efficiency: The ratio of the number of physical writes of a
  key block to the cache to the number of requests to write a key block
  to the cache in percentage. For a good performance of the MySQL
  server, the value of Key Write Efficiency must be 90 percent and
  above. If it is found less, then you can increase the size of the
  cache to improve the performance.


Key Buffer Used 

Find value of key_buffer_size as show variables like 'key_buffer_size';

convert it into MB.

Find the All MyISAM index Size :


  SELECT SUM(INDEX_LENGTH)/(1024*1024) 'Index Size' FROM
  information_schema.TABLES where ENGINE='MyISAM' AND TABLE_SCHEMA NOT
  IN('mysql','information_schema');


Find 

(Index Size)/key_buffer_size(in MB) * 100



  if result is (&lt;=100 ) then your all indexes are cached into key_buffer
  
  if result is (>100) then your all indexes are not cached into
  key_buffer you may gain performance boost by increasing
  key_buffer_size.


query cache hitrate


  Hit rate = Qcache_hits / (Qcache_hits + Com_select) * 100
  
  Insert rate = Qcache_inserts / (Qcache_hits + Com_select) * 100
  
  Prune rate = (Qcache_lowmem_prunes / Qcache_inserts) * 100