I have read where the block is used, but I believe the block or foot stool was 6".. It maybe more of your sitting posture in that you are leaning over your guitar to see where your fingers are going... I know of this problem because I am still trying to over come this bad habit that cause me back pain.. Also I have seen where a straight back chair is used...
Here a view of foot stool, but it doesn't give a height
http://www.sweetwater.com/store/detail/GtrFootRest/
A 2 or 3 pieces of 2x4 nailed together would probably work fine...
Good Luck,
Gordon