ATC college days...
Gosh this thread takes me back to what used to be taught at the ATC college...
Didn't the college used to say something like you can't go limiting an aircraft's IAS to less than it's Flight Level around the transition? E.g. For an aircraft at FL 290 then IAS 290kt, FL280 then IAS 280kt, FL270 then IAS 270kt, etc., etc.
(Maybe I wasn't listening too hard but it seemed an easy concept?)