You could try arguing that the 5' dedication is a hardship and that you should be allowed to count that square footage as part of your RFA. your plan checker can grant that depending on why the city wants the 5' dedication. Otherwise you are probably screwed. My next question would be, can you deduct that 5' dedication from your prevailing setback calculation to give you shorter front yard setback?
Bottom line (literally!): "subsequent" means subtract the five feet before calculating lot size.