Packing Box 2 - help with steps

When packing Box 2, I understand that you begin with the middle cavity by first adding 1 bucket (2 gls) of refractory and pack firmly. When filing the side cavities do I add 2 buckets (4 gls total) to either side, pack firmly and then 2 buckets (4 gls total) to the other side and pack? I’m assuming that since the center cavity is a lot smaller than the side cavities the 1 bucket center and 2 buckets for each side “levels” each step of packing? Repeat all this until all three cavities are full and firmly packed? Is my understanding correct?