I reckon there woul dbe lots of empty seats quite often at 60k myself.
That, in and of itself, isn't the end of the world as long as you have a smart ticketing system.
But i think 55k is the sweet spot personally
Need to consider cost of tickets/what we'll charge.... simplistic terms 30k stadium full at £60 a ticket gets you pretty much the same ticket income as 70k stadium at £25
Would hope we'd charge less to get more numbers in.... more numbers of course gives higher chance for ancillary sales of food, drink, kit etc etc