Refund under Inverted Tax Structure

My input credit @ 5% = Rs. 100 = Tax Rs. 5, @ 12% = Rs. 30 = Tax Rs. 3.60, @ 18% = Rs 15 = Tax Rs. 2.70 @ 28% = Rs 10 = Tax Rs. 2.80 

Input Service @ 18% = Rs. 40 = Tax Rs. 7.20. Thus my ITC is Rs. 21.30.

My Sale = Rs. 200 @ 5% = Tax Rs. 10. Export (Under LUT) Rs. 50.

How do I calculate my refund under Inverted Tax Structure?