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

c1 = Width of side bridge - long hole

c2 = Width of end bridge - long hole

e1, e2 = Width of plain margin parallel to long side

f1, f2 = Width of plain margin parallel to small side

l = Length of hole

s = Thickness of plate

t = Spacing

t1 = Side pitch - long hole

t2 = End pitch - long hole

w = Size of hole