the maths

A3 print = 420mm x 297mm
Gap = 55mm
h = (3 x 420mm) + (2 x 55mm)
   = 1260mm + 110
   = 1370mm
or 1m 37cm
w = (12 x 297mm) + (11 x 55mm)
   = 3564mm + 605mm
   = 4169mm
or 4m 16.9cm
centre height = 60" or 1524mm
top = 1524mm + (1370mm / 2)
      = 1524mm + 685mm
      = 2209mm
or 2m 20.9cm