If the ticket says "report by x", you want to be there by that time for check-in. I'd say half an hour earlier than that just to allow yourself some wiggle room in case you have trouble finding the building or classroom.
That's so sweet of you! Even though the rules are very apparent on the LSAC website, email, and ticket, many people forget this very important step and end up freaking out.
I just called the LSAC and the guy said the last "event" they had on my file was printing my admission ticket. There's nothing else there showing that I took the test. This is unreal.