I've been filling the 30 with short splits since that is what I have at this point in my wood stack. With the short splits I have been filling the right side of the firebox really tight E/W and then I do a row N/S on the far left side to completely fill the fire box. The short splits seem, for me at least, to eliminate wasting space in the firebox. I just got my second 12.5 hour burn cycle (temps at or exceeding 250 degrees.). This surprised me since these shorter splits are also smaller in diameter. I figured, if anything, they would burn faster. But, I guess, since I am fitting more wood in the firebox, it doesn't matter as much if the pieces are smaller.