Some very interesting numbers and proposals from the following article. The proposed 0.70% interest reduction in the eurogroup will reduce Greek debt only by 1.4% in 2020. But if Greece manages to stop or suspend paying interest in her EFSF loans the Greek debt will be reduced by 17% of GDP (about 44 billion, as it's the current tranche), in addition, if The ECB returns its profits on Greek bonds that has in its portfolio (its got a discount on purchases 25-30%) then the debt would be reduced by a further 5% of GDP (meaning that ECB so far made a profit of about 14 billion by "helping" Greece only with these particular bonds).
[link to translate.google.gr]

No worries though, Greek govt is negotiating very hard for the 0.70% reduction.