"Whats the lowest I would want the first port resonance to be....figuring the TD12M will be crossed somewhere around 1200 hz?"
for all practical purposes the resonance of the port in a main will be completely masked. by the time that the air velocity in the port is making any noise that you will hear, the main will be blasting out over 100db spl. if it makes you feel good, the port on parham's 4pi is 3.5" x 6" and in his cab that puts the first resonance right in the middle of the passband for the woofer, with nairy a complaint about resonance.
7-16 sq. inches of port area for a 12" driver is common, however you want to divy it up (1 x 3" round, 3"x3" square, 1x4" round, etc.).