Measuring IOPS for innodb_io_capacity
If you have a running instance of mysql, throw some data at it, then compute
( Innodb_pages_read + Innodb_pages_written + 
       Innodb_dblwr_writes + Innodb_buffer_pool_pages_flushed ) / Uptime

Something like:
SHOW GLOBAL STATUS;
run some heavy I/O queries
SHOW GLOBAL STATUS;

Tediously subtract the 5 values and evaluate the expression.
This will be a crude number, and you can probably set the innodb_io_capacity* higher than what the expression gives you.