Find the correct Boom distance for Lightning P-38 Recumbent

Its been raining non stop for the past 4 days and its not a ideal condition to ride. So I setup my trainer at my living room. To adjust the boom of the Lightning P-38 Recumbent. The first longish ride, I felt the distance was abit too short ( I move the boom all the way in). And this time I adjusted it 1 cm longer and tried out. It felt better but I am not too sure. Can recumbents and bike experts give me some advice? Thanks. 
Here's some of the video of the adjusted boom length.