I have ADHD too and would find a window difficult. I'd aim for 9:25. But a window of time provides me with options, which is bad for my time keeping.
I'd be bang on 9:25 as long as nothing disrupted my flow. If something did disrupt my flow we're in the territory where Im constantly texting you telling you exactly where I am so you can still know exactly when to expect me. I would also be very apologetic.
I'm a very annoying person.
I have missed loads of appointments too, so it's not just friends and family this happens to. My GP tells me a time to get there and they fit me in whenever now. My GP is amazing.
I've had therapy for this! This why I have a system for leaving the house now!