I baked one this size just last week and I only used the flower nail for the center and it came out pefect.
I would use one.. or a flower nail. I use a flower nail in pans 10" and up.
I don't use anything in the middle of my cakes either and they also turn out fine. You should be good either way 
Kat
Silly question but what is the flower nail or the heating core for I have bake a 10 inch, 12 inch and a 12x18 sheet cake and never used anything and they all baked even with no problems but i am new to this so I was wondering what they do
sometimes with large cakes the outside bakes much faster than the middle, leaving it gooey inside and burnt on top. with a flower nail or heating core in the center, the metal conducts the heat and allows the insides to bake evenly with the outside. i always use one on anything 10 inches or larger, and i use 2 on anything bigger than a 12.
i baked 2 8 inch chocolate cakes. i only have one flower nail, so i put it in one of the pans. the cake with the flower nail completely crumbled. the cake without, was fine. did i do something wrong? or was it just a coincidence?
It wasn't until I read here that you could use flower nails to help with the center of your cakes baking more even, I tried it. I was so amazed how much of a difference this made. I used to have a dickens of a time with massive crowns in the middle of my cakes, especially the larger 18" squares and such, but since using flower nails, not so much any more. I was happy to hear of this hint. You should try it to see if it works for you too. I love all the new things I learn here on CC!!! 
Flat side down on bottom of pan? It pulls out of the cake after baking with ripping the cake up?
Val
Yes the flat side is down on the pan. When you flip your cake out of your pan then the flowernail will be on top and you can pull it straight out.
HTH
OK SO THIS MIGHT BE A SILLY QUESTION BUT WHEN YOU USE THE HEATING CORE DOES IT LEAVE A BIG HOLE IN THE CAKE BECAUSE THAT THING IS NOT SMALL! JUST WONDERING...
If you use a heating core, place batter inside so when you turn your cake out, you can plug the hole with it.
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