over simplified - calculate length times width of the desired finished size (ie 106x106) to get total square inches of the top. Divide this number by 1512 which is the number of square inches in a yard to get total yardage in the finished top. Since you want to use two colors, divide your number by 2 to get yardage for each color. so 106x106=11236 divided by 1512 = 7.43 yards. Now, remember, you need to add more for what you lose in seams and waste. If it were me, I would buy 5 yards of each color for this size quilt. Borders? add extra. To calculate for borders, figure out the total linear inches needed for the border, divide by 40 and this will tell you how many cuts of fabric you need for the border. then multiply this number by the cut width of the border for total yardage needed.