use this : http://www.wheelpro.co.uk/spokecalc/
really easy ...
ERD=rim dia
offset= 0for this rim
flange diameters are same for this hub 56mm on both sides so type it to "Hub flange diameters" left and right 56mm
centre to flanges = 32mm both sides type it to "Flange distance". than click on calculate and you can see what lenght you need for radial,2x,3x or 4x crossing