Cenvat Credit

Hi
We are an IT/ITES company providing implementation services. We have been taking Cenvat credit and restricting it to 20% of our ST liability for the month.
I see that the rules were amended last year to 8% of of the value of exempted services.
Can you explain as to how this works? Does value mean value of sales made on which ST liability arises? Are we allowed to change it now?
An early reply would be appreciated...
Rajashree

Just to add.. all our services are service taxable ie., we charge service tax on all invoices raised.