For the length of the seam on the inner side of each leg, do not use any insulation. Since no insulation is used there is no reason to use a double layer of shell material. I suggest a width of this area to be 2.5 inches. You save the weight of down in this area and the weight of one layer of shell material. Or the down could be shifted to other areas of the garment. Less insulation in the inseam area cuts bulk, chaffing, abrasion. I usually use insulated pants to sleep in, and since I use a mummy style bag my legs are together and need less insulation.