Requesting work time reduction to 80%


In the next few days, I am having my appraisal meeting with my boss. I want to raise my desire to work 80% instead of 100%. I know from former discussions with him that he is not a fan of part time and he will find many reasons not to accept it. I find this quite unfair since many of my colleagues work part time, but colleagues in my position (all female and no children yet) are not allowed to reduce working hours. Last week, I asked myself how I want to spend my time and I realized that I want to have more time for my private projects and stop working 100-120%. My question now is how can I go into this meeting in a constructive manner and keeping the balance of being clear enough in my request and accepting his decision.