Be carefull when drawing in free hand. 18 pixels is 2.74 meters, and 17 pixels is 2.5908.... the difference, roughly 15 centimeters (0.5 feet) multiplied in 10 times (ten decks) bring out an cumulative error of 8 pixels (thats half of a deck!) to the all meassures. Thats why I always recomendate to use either tracing technique, or having some real and accurate plans around you to make proper calculations to each measurement. Drawing accurately with free hand needs actually lot more math skills than drawing skills