What you need to be asking is what load centre you are working at. If you are working at 500mm then a 125 x 45 section fork in "industry normal" material has more than enough capacity to match your lift truck capacity. If you were to use a fork with an "optima" heel then you would have a thicker wear zone than a standard 50mm fork and then the difference between 45 and 50mm thickness is not relevant.