Eyestrain

Eyestrain is a biometric application helping you find and preserve the right position while working with your Mac. Using computers for extended periods can cause visual discomfort, headaches and vision challenges: that's why we created this application!
Eyestrain is as useful as intuitive to use!
Once you are positioned between 18'' and 24'' away from your monitor, as recommended by Apple, (http://www.apple.com/about/ergonomics/vision.html) save the optimal position.
Then, whenever you want, check if you are sitting too close.
Don't forget to set a reminder too.

It will invite you to check your position periodically to make you aware of the best position everyday more.