Actually, after a bit more playing with it I decided to go for something rather more sophisticated. Rather than having a simple 3x3 grid dividing each square into corner, edge and centre regions (left), I went for a more complex model (right) which expands the edge regions at the expense of both corners and centres:
That seems to me to be generally better all round. The version of Rectangles now on my website has been updated to do this; give it a try and let me know if it's any better?
*grin* Two people actually asked for the flash - they'd got used to it in some of the other puzzles and missed it in Rectangles. I thought this one was already obvious enough when you'd completed it that I needn't bother, but no, it seems once you give people one flash they're addicted :-)
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That seems to me to be generally better all round. The version of Rectangles now on my website has been updated to do this; give it a try and let me know if it's any better?
And I love the way it now does a little flash when you complete the puzzle!