Oh dear, double maths lessons all round.
500 mm x 500 mm = 2500mm sq mm = 1/4 m2
so for 500 mm (or 50cm) square tiles its length times width divided by four, not two to convert to m2
Or as I badly explained ,, walk across an area count the tiles in one direction , eg 100 , divide that by 2 =50 then count the tiles in the opposite direction eg 40 divide that by 2 =20 then work out 50x20 (or because I am simple 5x2 , then add a zero) =100m2 ,
whilst I am doing this most customers will ask why I am staring at the ceiling , and then I launch into my measuring method , they then smile , or are taken off guard , and rapport is established.