cant see what the problem is, you dont need to 5spot your tiles, please dont tell me that you notch stone! you say that you use a rubber mallett next to bang them down to level?
get yourself a some pourable white, Bal do one, or even nicobond white or granfix marble fix or equivalent, mix up the adhesive runny so that you can pour it, but not too stiff that you cant push the tiles down. find the highest point of the room, theres your setting out point or a reference point as to how much gear you'll need to build up with elsewhere. pour out your bucket of creamy adhesive, run your trowel thru it to make your hills and valleys, put a runny blob in each corner and one in the middle of where the stone will sit. put your stone in place, push down, the adhesive runs out of the sides,you should have a solid bed underneath with the corners fully supported, get it level.get a couple stones down with the required bed underneath, get them level. now you just have to work backwards checking the level, more or less adhesive needed, just pour more in, doesnt matter if the floor is unlevel, because your floor is. this technique overcomes the need for levelling and ensures a solid bed underneath (no corners breaking off). So,the builder dont want to pay for levelling, but he'll pay more in adhesive costs one way or the other whether the floor is spotted or not. tell him you'll pour your adhesive to get the floor level, allow 20kg for 2/3msq depending on how much out of level the floor is. then tell him the cost for levelling. up to him then. simples.
