Bike computer wheel size setup
Look at page 23 and 24 of the manual and follow the directions for measuring tire rollout. Measure your front tire and select the wheel size on the chart that has the closest measurement to your tire. There can be a big difference in the circumference of different brand/model tires even if the stated size is the same. Measurement of the actual tire circumference will give you the most accurate readings. As far as placement it makes no difference. As long as the magnet on the spoke triggers the pick-up it will count revolutions. The computer does the calculations to convert the data to mph, distance etc. My preference is to mount it as close to the axle as possible. On a mountain bike this keeps it out of the mud and away from branches.