I don't get why some people are so rude, if you are not going to help the guy just move on, no need to bring your saltiness.
Anyway, as you probabbly know, optimal speed will be dependent on the air direction and speed, so do like the real pilots do and go with the motors RPM, if you switch to radar mode it shows an arc to the left, it shows your motors RPM from low (blue) to high (red), I think it also shows a numerical figure but could be wrong about that last one, anyway stay in the green and it will give you the optimal speed on whatever wind the aircraft is.
On low wind conditions, if I remember correctly, the optimal speed was around 40-46 kph