Let's see, 12 strand rope, cut one strand each 12 inches. That's 12 feet of tail? Seems quite excessive!
NE Ropes recommends a tail of about 28D for doing a brummel splice. My experience is that the eye to eye length comes out pretty close before doing the bury, and with 30D of untapered bury (not recommended) I lose about 6D - less if the tail is tapered.