Are you satisfied with the mark or score you award pupils in composition writing?What makes you arrive at it?

No, system is stronger. 

Do not forget that teachers are always mistaken NOT the students!

After their first attempts at a composition I can set an "average" standard for future work. A critique of their initial offerings will follow with advice on where I think improvements can be made. Once this standard or mean average has been set I judge and mark, above and below the imaginary line. Marks are afforded for the various significant elements, layout, formula, grammar, creativity and imagination all based on whether in my opinion, taken individually, these elements can be assessed as above or below the average. This method lets me see where there are areas for improvement in as much as someone who has written an imginative composition but is still in the average bracket needs remedial work possibly on one or more of the other facets, maybe grammar. Someone who is grammatically correct with a well laid out paper but, again still in the average bracket possibly has a more limited imagination or sense of creativity and so on and so forth. It will only be a matter of time before you can determine the standard your students have reached and which areas need to be worked on, if any!

Yes of course,There is a grade for each correct sentence and a grade for each wrong sentence according to what is right and what is wrong.

Yes, of course there is a degree or mark for each correct  or wrong answer.