Originally Posted by Canonet
Oops -- should read "Lower back pain might be caused by too little space between your hand position and your saddle, rather than too MUCH."
I agree,
Here's another one of those rules of thumb that I can't explain, but seems to hold water:
When you are in position with your hands on the hoods, your handlebars should line up (block the view of) your front wheel axle. If your see your bars behind the axle, that may be one indication it's too short of a reach.
If so, try a longer stem. But if everything is sized properly, your stem should probably be between 90 and 120 mm.