On top-loading kilns, placing a shelf too close to the top of the firing chamber can lead to over- or under-firing.
Suggestions for placement of the top kiln shelf:
1) Mount the top shelf low enough so that element grooves show between the kiln lid and top shelf.
2) Place tall ware on the top shelf. This necessitates lowering the position of the top shelf.
3) If the ware on your top shelf is over- or under-firing, try using two half shelves instead of a full shelf. Stagger the height of the shelves.