Based on your countdown clock, it looks like you are arriving around the 13th...
Your question is difficult to answer because of all the schedule changes and cuts being made by Disney.
First and foremost, the fireworks were only on Saturday's in May last year, until Memorial Day weekend. I doubt they will be any more frequent this year.
If we go by last year's schedule, the fireworks were at 10pm on the 11th and 18th. But with hours being cut so much vs. last year, there's a good chance they will be as early as 8pm or 9pm. I doubt they would close MK any earlier than 8pm on a Saturday in May.
Make it for any later than 7:30 and you run the risk of having the fireworks happen before your reservation.
Any earlier than 7:30, and you increase the risk of having to wait around after you eat, in the event the fireworks don't happen until 9:30 or 10.
I'd probably go with 7:30, then wait for the May hours to be released. Even then, I don't know that I'd try to time it perfectly, because the schedules could still change.