My experience is that a programmer will spend about 25 percent of his time on analyzing and planning a project or program. He will then spend about 50 percent of the time on a project in actual coding, which is simply translating his solution to the business process into one or more source programs in an appropriate programming language. The remaining 25 percent of the project or program time should be spent in very careful and comprehensive testing and system testing with representative test data that really exercises all of the known and anticipated program functions. What you should do is provide your manager with testing that includes the details that prove the correctness of your changes, not just your answer.
The savvy programmer becomes super-productive by copying not only huge blocks of source code, but also single source statements that he needs. So, do the smart thing: copy, copy, copy (not to mention test, test, test). And never try to wing it.