There are several fit methods that exist where either inseam length or leg length is the measurement that is used to determine seat height based off an equation. Where as this is an option that could help you determine a reasonable seat height we do not feel it is an optimal, or even a viable option for finding your seat height.
When initially hearing the thought behind these methods they make sense. Your leg extension is the primary factor in the equation of saddle height and based on a fraction of this you could feasibly create an accurate saddle height.
Upon greater inspection and comparison with motion capture, of how individuals interact with a bicycle it has become evident that there are many factors not able to be considered when solely using inseam length/leg length. These factors include the three key angles (ankle, knee & hip) that vary from one cyclist to the next, how you determine where you sit on the saddle as well as pelvic rotation. With these factors, and a firm understanding of bicycle fit, we find it very difficult to believe that one simple measurement could take all of these factors into consideration.