Hi ken,
I think that debt of 90% of GDP (alhtough some say 100% is the worrying point) is a historical thing. Once you reach that point, historically, there is only one way to go (although from what I have read, 100% is often used).
I s'pose the reason is that once you reach that level of debt, simply paying the interest on such debt reduces your ability to do other things; which starts a downward spiral.
They're my 2 cents worht anyways.
- pug
