I always use a 12×18 for my sheet cakes. I used to use flower nails and they worked great, but don’t feel the need any more. The best advice I can give is to use parchment paper. The biggest issue I used to have was it sticking in the middle, no matter how much I greased it…plus I hated using so much spray on it. The parchment paper makes it SO easy to turn out and doesn’t make the bottom greasy. I line the bottom with parchment and fill 2/3rds of the way. Before I bake the cake I place it on this large sports ice pack I have for about 5 to 10 minutes. (I did that once when I had made the batter but had to wait a bit longer for another cake to finish before I could use the oven. It made my cake bake out completely even and I didn’t have to level at all so I have done it since then). My edges are never hard and it bakes up nicely. Hope this helps.