firebrick clearance options. help?

[cjf12]

I have the firebox bricked and am now at the drop flue pan. I have 1 inch ceramic blanket already in place.
Here is what i feel my optuons are.
1. Go with full brick and have about a 1/16 of a gap between flues and brick. Could give a little more room once pan gasket is in place but then with weight of sap in pan will it compress anyway causing flues to be too close?
2. Go with 1/2 brick which will give almost 2 inches to bottom of flues.
3. Just fill it with sand to just below the flues and save some money.
How close should i be?

[Sugarbush Ridge]

I filled with sand to get split brick to less than 1" below flues. #1. I feel that the brick keeps more heat under flues. #2. I brush flues every day,,, the brick keeps for dragging groves in sand and then actually have sand up against bottom of flues. #3. Blanket only,,,, pulling it out when brush,,,,, how much??? but would seem some would be pulled out

[maple flats]

Sand is an option if not needing to add too much. When you need a lot of volume filled vermiculite is better. You want about 1/4-3/8" under the flues to force the heat up into the flues. You also want a layer of brick just under the flues so when you brush the flues you are not disturbing the surface under the flues.