I have run calculations using a joist calculator which have said that 72x170 C24 would be sufficient for my 4.3m span but I cannot find this timber locally so I am trying to work out if 47x175 C24 doubled could be an alternative? The calculator said 47x175 C24 would be okay for 4m so doubled could it go the extra 0.3m or how can you calculate how much further it can go please as I cannot find it anywhere?