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You should probably just accept it, then request another one in 6 months. Only decline it if you think you can get a better CLI sooner than the 6 month waiting period.
This time around, take whatever they offer as it will be your only shot to request before the $500 increase happens.
In the future, you'll want to decline whenever you deem their offer to be too low. Accepting the offer would kick in the six month waiting period. Declining would allow you to try again as soon as you think you might be positioned for more.