There are a couple of reasons for using the Constellation class. The first is that it is the most recent adaptation of the FREMM. As you stated, the analogue approach is based on the Arleigh Burke vessel. This mirrors the approach of the Congressional Budget Office. It is a very good analogue, once proper adjustments are made. It is a general-purpose frigate, which is what the type 26 is intended for.
I should also note that we're not using the CBO estimates for our own estimates. We're not using their estimates as raw data; we are using the Arleigh Burke as an analogue. It is thus a similar approach to what the Congressional Budget Office undertook.
I believe that answers your question.