the key is to find the sale amount for the similar homes (not the assessment amount).
when you file with the board, mpac is obligated to provide you their analysis which will include all sales they are used for your assessemnt.
you have to file with the board, otherwise you will never win, because mpac rarely "re-consider". they only "re-consider" (settle) days before your board appearance.
also read ontario ombusman report..
note: MPAC has been ordered to clean up their act for next two years.