What percentage to settle on a charge-off account?

I have several charged off credit card accounts. I am trying to clean up my credit. I did read on your site about the "Pay to delete letter" but what percentage is recommended? These accounts are about 3-4 yrs old, all have been sold to collection agencies ranging from 500-2000$$ majority of the amount owed is overdraft and late fees that kept adding up every month until it was charged off. I don't have much $$ to work with but wanted to take a portion of my tax return to improve my score since I plan of moving at the end of year and need a credit miracle. Thanks for your time.