It has nothing to do with discounted tickets. I haven't done the math myself, but I'm betting ACL got the seating capacity from the grosses (which does not include standing room) and then multiplied by the attendance percentage to get 26 standing room spots.
Actually, I just went to do the math myself and it's even easier than that. BWW grosses reports a maximum weekly seating capacity of 8,528 and the number of sold seats as 8,736. That means they sold 208 more tickets than they had seats. Divide that by 8 performances, and that's 26 SRO spots per performance.
It's possible that obstructed views are not included in the capacity (I know some small theaters don't really count them), so it might be closer to 20 SRO spots.
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