Okay, I've been doing cakes since I was 16 and never heard of a cake baked at 250 degrees. I bake cakes at 325 when they are large.
To keep a cake even, keep the sides cool as it bakes and make sure the temperature reaches the center. The sides are kept cooler by wrapping them with the bake even strips or towels strips. Soak the strips in cold water and remove the extra water before placing around the pan. I run the wet strips between my fingers as I do not want to wring them dry. Wrap the outsides of the pan and pin in place.
The cake pan is greased and I line the bottom of the pan with parchment paper to assure the top of the cake does not get stuck. Since you are making a large cake because you are using 2 mixes, take your flower nail and grease it up. Place it in the center of the pan with the nail part up.
Pour your batter into the pan and around the nail. Fill to 2/3 full. I tap the pan on the counter to get the trapped air bubbles to rise to the surface. Let the cake sit while you preheat your oven to 325 F (about 10 min). Before placing in oven, I pop the visible bubbles with a clean toothpick.
Set your timer for 1 hour. Check after an hour with a toothpick. If it comes out clean it is done. If not keep checking every 5 minutes until it is does. Remove from the oven and let it set a few minutes. Turn out onto a cooling rack and let cool. Dont't forget to take out your flower nail from the center!
As for other size pans, DH used to print directions from Wilton as to how to bake on the inside of the mix box. Not all of the boxes seem to have this info so I am attaching a copy. Also I use the nail trick on rounds over 8" and from 9 X 13, up.
I hope this he