With rail seats I believe technically the ratio between seated capacity and standing capacity can be different, but obviously depends on our regs.
1:1 is the current norm here but I believe years ago Everton were pushing for 1:1.5 in their new stadium, so eg 5,000 seated mode vs 7,500 standing mode.
I say seating vs standing mode but at the moment this country has a pathetic form of rail seating rather than the lockable, flat surface ones abroad.
Fair point Lark re: the roof. I was assuming a greater capacity than currently as we’re only using the front half of the BMS, and with no home fans in it.
Maybe you could have a smaller version of the current full-height BMS but with a roof, but then it would still impact certain houses in the GRS.