After my weekend photoshop workshop last weekend I remembered this Photoshop curves trick.
It is a great trick I learned a long time ago, but, I can't remember where or how I learnt it?
This is the usual curves dialog box, after I have reversed the values by clicking the small black and white triangles at the bottom of the graph.
If you hold down the option key on the Mac or the alt key on Windoze, hey presto you get 10 squares vertically and 10 square horizontally.
Divide 255 by 10 gives you 25, this then enables you to loosely apply zone System ideas to your images in Photoshop.
You can even make a tool, that can help you visualise this all by making a strip that has 11 shades of grey from 0 to 255 in it.
Another little tip is if you click the small button on the bottom right of the dialog you get a large version of the dialog box.