Using this code:
x = x + (canvas.height/250);
which happens every 1 millisecond should add an amount to x in proportion to the canvas size. x is then drawn so therefore x should move down the canvas at the same speed on different screen sizes (the canvas changes size according to the screen size). However the x moves down at different speeds on my ipod and my pc.
If you want to know the full source code and html file the html is here and the javascript file linked to it is here.