Gordons reloading tool, what data points are important and how do you enter the gas port length for calculating semi auto?

[Markbsae]

I have downloaded Gordons Reloading Tool and have been working on getting it down. Lots of data points to enter. I did a web search on calculating for semi auto and cannot find the function to enter gas port length to work on semi auto (AR). As far as critical data points, I have measured true case capacity in grains of water and have entered OAL and jump to lands. Lastly, when taking calculated vs actual velocities what function is used for truing actual velocity? thanks