My Sister in law gave me a set of Speedplay Light action pedals for my new road bike. I have SPD pedals on my Hybrid and never really liked them. I also got a pair of Speedplay ready shoes and when I attached the cleats to the shoes I put them dead center. However, I see there is a fair bit of leeway available in placing them on the shoe. Is there any rule of thumb for where to place them front to back or is it just trial and error based on comfort?