1.2 is probably just the length of the loaded rounds they tested with, not a maximum (or a minimum.) Did they use exactly the same bullet? If so, you should strongly consider using that length. If you're using a different bullet, load them where you think they need to be, and take 1.2" under advisement when adjusting the powder -- if you load them a lot shorter, pressures will be higher so don't go all the way to the max. If you load them a lot longer, maybe don't start all the way at the bottom for your minimum load. But it's really the seating depth that matters for getting the powder right. The OAL has more to do with feeding and chambering reliably.
Of course the OAL and the seating depth are related, but it's based on the length of the bullet. That can vary quite a bit if the nose profile is different.
"The only way to stop a bad guy with a gun is with a good guy with a gun"