Depends on the popularity of the show. If it's not too big then I'd recommend 6:30 but if it's a more well known act then get there at 6pm or earlier because there will be a huge line outside the venue.
It is a large venue with 2 balconies and a wide stage, concrete risers in the back. If by "good spot" you mean on the rail, get there shortly before doors. The parking nearby the venue fills up much faster than the venue.
Don't forget to budget security line time into your arrival time. We've been thru a few times and it averages about 15 minutes.
By 7pm
Cruise in at no later than 7 and you should be great. Note they have some Happy Hour specials, 25% off and $5 Pacifico's. Enjoy
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