Laying out tiles

[justaguy]

We've just completed adding a utility/shower room to the side of our 1930s semi, and Mrs C has decided she wants the family room/kitchen/diner/utility/shower room tiles.

N&C in Colchester have been helpful, and I've now got 350+ 400x400 Porcelain tiles to lay but I'm struggling with working out the best way to lay these out.

Since it's a largish open plan space, and I'm a bit OCD, I thought I'd pick a focal point and do a 'dry run' to see how the cuts would work out before I started fixing them properly.

Mrs C has decided the focal point should be the fireplace, so I lined up the tiles on the hearth and ran them out from there only to discover that the walls are out and not by a little, but quite a lot.

Not sure what the correct term is, but I'll use the term 'runout' as using the fire hearth as a starting point (focal point) I'm looking at cuts starting at 5cm but ending up as nearly 8cm just 4m away and that to me means that it's going to look 'pants'. From that, I conclude that the fire heath is not at a right angle to the adjacent wall.

But f I use the wall as my starting point and tile out from there, I'll have a difference of 2cm in tile cuts across the front of a 120cm fire hearth - equally pants.

I'm wondering if I'm making this more difficult than it needs to be.

I've attached a sketch of the space to be tiled, and would appreciate any suggestions as to how I should address this.

TIA.

Martin