Vat input reverse on stock trf. (tn state)

In the state of Tamilnadu, what is the rule of input credit reversal? Suppose in the partcular month, total local purchases made for Rs. 1000/- plus tax Rs. 50/- ( @ 5%). It means a dealer is eligible input credit for Rs. 50/- if it sells within state. If it sells locally (Rs. 20%), centrally (25%) & also sending to branches (55%) out of state against F form then how much will be input credit. I know that I am eligible input credit on sales (Locally & Centrally)  but not eligible input credit on stock trf. out. My question is that what is the % of tax (2% / 3% / 4% / 5% or else), we'll not get input credit or we'll have to reverse input credit if input credit taken in earlier month. Pls. reply with rule.