I haven't done this, but I have several people who report to me at work who have. What I appreciate, and what makes me most likely to approve this, is when the person comes in with a realistic list of the expectations of the job, and a fully planned out schedule showing exactly how he/she is going to meet those responsibilities. And, I ask the person to give me a calendar view printout of the schedule so I know exactly when the person will be in the office. This is important, because if I am planning a meeting that the person needs to attend, I need to know what days/times will work and I don't want to have to check with the person every time. If I have a question for that person, I want to know when that person will be at his/her desk.
So, what exactly is your 40 hour week going to look like? Don't just say "I'm going to need to take an additional 1/2 day sometimes". Have a solid schedule and stick to it, eg "To make this work, I'll need to leave at 2PM on Thursdays instead of 5, but I will make up those hours by coming in at 8 instead of 9 on M, W, and F." (or whatever) And stick to it.