When i was making "frankenarrows" more regularly, i would buy odd quantities here on the TG, the big auction site, or anywhere else i could buy them without paying a premium.
My usual footings were 1-3 inches depending on how much i needed to stiffen up the spine, and also if i wanted to up the FOC a bit. My experience has been that aluminums respond to footing as much if not more than carbons, with an inch of footing behined the insert adding approximately 4-5 lbs of stiffness.
I'm actually digging through my boxes to build a few for a goose hunt next week. I'm hoping to find enough pieces to build some 2219's footed with 1 1/4 inches of 2419 and some big natural turkey flu-flu's. A few 300gr broadheads and i should be good to go.