Aperture drops normally are placed much higher in the kiln than most work to get the greatest length of drop. This means that the glass is near the elements and so will be heated unevenly. It has been said that the heat evens out across the kiln approximately the distance below the elements that they are apart. So if the elements are 100mm apart, the heat will even 100mm below the elements. This constraint means that it is difficult to get the length of drop wanted and still have the glass heat evenly.
There are at least two things you can do to get more height. One is to take out the shelf and its supports so you can fire on the bottom of the kiln. This will give up to 50mm extra drop length.
The other is to go ahead and fire closer to the elements than is indicated for even heating. This will require radical modification of the heating schedules. [qv firing]