Continua me dando coordenadas erradas.
<canvas width="600" height="400" ></canvas>
<script>
var tela = document.querySelector('canvas');
var pincel = tela.getContext('2d');
pincel.fillStyle = 'grey';
pincel.fillRect(0,0,600,400);
function exibeAlerta(evento) {
var x = evento.pageX - tela.offsetLeft;
var y = evento.pageY - tela.offsetTop;
alert(x + ',' + y);
}
tela.onclick = exibeAlerta;
</script>