The increase in the cost of living is not a percentage of a person's income, but a percentage of the cost of the basics in life. By calculating a COLA as a percentage of income, some are awarded money for luxuries while others don't receive an increase big enough to cover actual needs.
The federal poverty level income isn't really enough to live on, but twice that certainly is. Using a percentage of double the federal poverty level would provide an amount certain to cover actual increase in the cost of living, but not provide luxuries.
COLAs should be reformed by ending the present calculation and replacing it with a flat amount for everyone, based on double the federal poverty level.
Savings from this change for all federal payments should total tens of billions of dollars a year.