Stone tiles are cut with a diamond tipped saw which can be between 0.5mm – 0.75mm thick. Therefore whilst a tile on paper may be declared as 400 x 600mm there will always be a small amount of variation from tile to tile. British Standards declare an acceptable variation is + 2mm / -2mm across a floor. Therefore assuming a tile is cut square ( ie both sides are parallel it is possible for a competent installer to lay a floor with a 2 – 3mm grout line. It is impossible to get tighter than this due to the variations described above.