Effective Reach on Bike Frames

I just measured my Look frame, and effective reach is around 38-39. This means that my current frame’s effective reach is about 1-2cm longer than Cervelo 53. Two questions - What is the best way to calculate the effective reach? Are there any websites that provide information on various bike’s effective reach? Some manufacturers (e.g. Cervelo and Look) post this info on their web sites, but not all do.