The ratio isn't based upon how many people you can fit on the terrace, its based upon the amount of people you can safely fit on the concourse, and have the amenities to service.
At existing grounds, such as Goodison for example, you could add another level onto the park end, double the amount of seats and have steps leading up to the top. You wouldn't be allowed to let people sit there. Apart from it likely having fairly suspect structural properties, you can still only get amount of people in eth concourse it was originally designed for.
The site at BMD is constrained. We could design a ground that has concourse space for ~60k fans, but only seating for 52k, assuming Rail seating in areas can go to 1:1.8 (as Dortmund, with 2 standing rows per seated row). Or we could have seating for 55k fans, with concourse space for 55k, meaning that rail seating can then only be 1:1. As a person standing in an aisle realistically takes up approx 545mm width, while a seat takes 465mm width, the actual realistic ratio would then be slightly below the seated capacity, at about 1:0.8.
These are the issues facing existing grounds, concourse and amenity provision, not number of seats.