Hi Aaron, Sorry for the delayed response.
You are close.
Yes, you do want to stack the billets in the middle of the mold. However since you are trying to cast a relatively thin slab I have some other recommendations:
I recommend covering the open face of the mold with the billets, but keep billets about an inch away from any side wall of the mold. This way they will settle and flow outwards toward the walls rather than scraping down the walls as they begin to soften and flow. If they are in contact with the wall there will be a lot of sharp hang-ups that you will have to coldwork away.
Spreading the billets out to cover almost the entire open face of the mold will give you a clean casting, but will not require an extra long hold at your top temperature due to the billets needing to travel very far (horizontally) to fill the mold. Glass will assume a 1/4" thickness at casting temps pretty easily. But it doesn't flow horizontally very fast (hence the long hold time when stacking billets right in the center of your mold).
As for firing schedules: The schedule listed in the Lost Wax Tipsheet is appropriate as long as you are present to make sure the mold fills and the glass has flowed to where you want it. You will also need to adjust your annealing profile to suite your thickness. Check out our Annealing Thick Slabs chart. You can find it on the education page of our website.