Forget about the grouping comments as they're not germane to your installation.
You could use or the 10' or 25' tap rule. The 25' rules means that you need tap condcutoars that are at least 1/3 the ampacity of the OCPD ahead of the tap. If you can keep your tap conductors down to 10' or less than you only need to use conductors rated for 10% of the OCPD ahead of the tap.
Yes, but really, if he has a 1200A MDP, we can ASSume the main OCPD is no bigger than 1200A, so if he wants another 300A feeder, he is ALREADY at 25% of the OCPD. So the 10% issue is moot, but the 10' rule does mean the conductors could be just rated for the 300A, not 400A.
wbaldie;
The grouping issue would have been that IF you tapped off of the transformer, and assuming there is a Main CB on your MDP and it is the Service Entrance, then if you tap AHEAD of the MDP, you have to follow the "6 hand rule" for service disconnects, meaning they must all be close enough to each other so that a fire fighter can move his had only 6 times max to kill all power sources. Irrelevant however if you are DOWN stream of the MDP main breaker.
Which brings up an unaddressed issue; DOES your MDP have a main breaker? Or is is an MLO panel with no more than 6 feeders in it? If it is MLO and it IS
6 feeders already, you cannot do this at all.
Next issue: you are increasing your load by 25% of the maximum rating of the MDP, which is already full. Are you sure you have that capacity?