Yeah thats as good point, the walls could be a shambles and he'll need a ton more than what you work out going by the sizes. Anytime we've done tiling we've always resheeted the walls to sort this is issue out.
I think were all playing devils advocate here, I recon with the size of the room he needs like 2/3 tubs so seems like they are way off and as the OP said its a new room so they shouldn't have any of the issues with the wall being miles off.
Yeah thats as good point, the walls could be a shambles and he'll need a ton more than what you work out going by the sizes. Anytime we've done tiling we've always resheeted the walls to sort this is issue out.