Using honest JSON parsing.

Instead of "Pick the 3rd character in the JSON string".
Added a grid overlay.

Subclassed UIView with GridView and use that to draw the
grid and show on top of the painting surface.
Some considerations:
Not "optional" yet, just there all the time.
User can see the fudge factor on right and bottom.
Added ability to toggle the grid visibility.

With a boring little button.
An iOS master. I made the button toggle text.
Centered the grid and drawing view so is evenly divisible.

I mean, the natural screen size isn't a perfect square, so
now it's divided into the largest perfect square possible. And
centered.