Keyframe boost
i dont understand very well this option from the guide i always leave it a 40 but can i get a better quality/compression by changing it to other value ?

It's best to leave it at 40 unless you have a very "different" source that uses lots of fading scene changes or is very light/pale or very dark, in which case you may want to raise it. There will be very, very little gain by tweaking this option--even negative gains sometimes!-- unless you're sure you have certain "problem" sources, or the keyframes in your output video occur too often or not enough. Usually you'd want to raise it when you set max keyframe interval higher (but no more than 50).