I use a flower nail in all of my cakes. If it's really large, sometimes I use two. Good luck with your sheet cake! 
Hi,
Yes....in my 12 x 18 I use 2 and it works great.
Ues the wraps around your cake pan. They have long ones and you can wrap them around and pin them in place. It helps it rise more evenly. If you happen to get one sided hump, as often happened to me in my funky German oven, trim off the hump and keep the scraps to boost up one side. Just tuck them under the flat side until even and ice as normal.
You can also use the nails, but use 2, one on each side. 
so your saying that if I put a flower nail in there, I wont have the uneven hump that I have to cut off? I always end up cutting too much, then cutting uneven, and then cutting it again uneven... Its awful! LOL
and the parchement paper.. you line the bottom of the pan? do you go up the sides? I just use bakers joy. it works, but I am always worried I'll use too much. ??
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