It depends on the radar used. If the radar was mounted in the vehicle, it is mounted so as to compensate for the relative speeds of the two vehicles.

Be careful on your written declaration lest you admit to guilt, albeit at a lesser speed.

You might consider seeking the officer's notes as part of any discovery request to see what it is he is likely to testify to in court.

Also keep in mind that if you go to trial (and maybe even the TBWD) the court may decide not to allow for Traffic School if you are convicted. This option i most often guaranteed in a not guilty plea (assuming you are otherwise eligible), it may not be allowed if you lose a TBWD or a trial de novo.