I usually book for about 5 or 5.30pm even. If I book 5.30pm I will show up 20 -30 mins early as I want to request a waterside table and that can be a 20 - 40minute wait on average. I haven't waited longer then 25 - 30mins for the waterside table. I want to be done with our meal by 7pm.
Last time my son and I did it we lined up at the show 'entrance' area from 7pm for the 9pm show. You definitely don't have to be there that early but my son wanted front railing and he was prepared to line up for 2 hrs to make sure he got it. They actively discourage people from lining up that early but the couple of CM's that were there were happy for my son to hang near them and wait till they started the line. I just sat back a bit on a planter till the line formed.
Make sure to book the reservation right at the 60day mark. The prime dinner times from 5 - 6.30 is are what I find go first. Try making your booking for the 5pm or 5.30pm - whichever you prefer. When you click the time and go to the next screen it actually says up the top which showing the reservation is for: 1st or 2nd show.
If it's 2nd show then try making a slightly earlier booking to see if it will give you the first show.
I was fiddling around with it a couple of weeks ago researching for our upcoming trip in July and you could definitely still get Dinner - 1st show reservations - I don't know if there is an official cut-off time before it becomes 2nd show but I think booking at the 60 day mark right when they open the reservations for your date is essential.
