a0 = Relative open surface
a1 = 1st external dimension of plate (width of plate)
a2 = Dimension of perforated area parallel dimension a1
b1 = 2nd external dimension of plate (length of plate)
b2 = Dimension of perforated area parallel dimension b1
c = Width of bridge
e1, e2 = Width of plain margin parallel to long side
f1, f2 = Width of plain margin parallel to small side
s = Thickness of plate
t = Spacing
w = Size of hole