Thank you for yur answers.
Paul and pdeeh thanks for the link, I read it and it actually confirms what I wanted to do.
Thomas, I see what you mean. I know that I will have to increase the dev time whatever happens because I don't like printing at the limit grade 5, and having no room for increasing contrast if I want to. But I wanted to do this test, to check if my developing is too short or if the cause of the low contrast is eslewhere. Anyway I'm having fun with these tests for now, and will have even more fun printing from the negatives developed to my taste.