I've been noticing that my cakes are shrinking, quite a bit.
I use Magic Line pans, with release used to use Wilton's and switched over to CK Products. I don't use much of it at all. I use the baking strips along with a flower nail in all of my cakes no matter what the size. I have a "Speed Bake" convection oven. I set the temp for 330 and it usually takes 55 min. I noticed at 50 minutes I was still getting a little gooey not quite all the way done cake so I upped to 55 min and they seem to bee find now when I level them. So that is what I am doing, when I take the pans out of the oven they look a ok. In the cooling process they are shrinking quite a bit. I think the last 12 x 18 I did ended up being 11 x 17 1/4. I've noticed it with my rounds as well.
I've searched the forums and if the answer is out there I am obviously not finding it - so - What on earth am I doing wrong or is this normal and why everyone seems to use Earlene's chart vs the Wilton chart?
Only guessing, but it sounds like the cakes may be overbaking. the magic strips help, but for larger layers, an inverted flower nail will help the centers bake more even with the edges. For very large layers I use more than one nail. It works great.
330 sounds kinda high for convection which I realize they came out fine at that temp before--but maybe diddle with the temp--or part way through take it down a notch so it continues to bake but not loose the steam inside that is causing it to shrink later. But 330 is high for the convection ovens I've used.
Keep in mind, home based here so that is the type of oven I have. From what I have seen when I did try it was that if I had it at 325 and left them in for 50 min the top didn't seem quite done. This is even on 8 in cakes. 8 in shouldn't take an hour to bake should they, I mean that seems kind of long. Maybe I'm wrong.
When I do the larger pans I use the nails along with the strips that is why I couldn't understand the shrinkage.
Try a tent of aluminum foil over the top after it's been baking for a while and leave it on even after it comes out of the oven--will increase the heat to the top a bit.
Check Wilton's chart to see how much batter they recommend that should go in the pans--this makes a big difference too.