Mvat on purchase of property

I got it govt. web site 

https://mahavat.gov.in/Mahavat/MyFold/DOWNLOADS/NOTIFICATIONS/KNOW_NOTIFI_MVAT/KNOW_NOTIFI_MVAT_01_29_14_6_23_51PM.pdf

My builder sent me VAT notice for transaction in period 2006-10.   In his letter he mentioned that the details VAt calculations cannot be given to you.  I havetoappoint some CA and he will give details of VAT only yo the CA assigned by me notto media tly.

As per his calculations I have topay VAT of Rs. 1.2L. For a flat of 25L (I guess 5%)..   But during registration I did ADF and paid 100% .  The stage of construction at that time was 80%.   As per this GR I am falling in forth stage and my VAT calculations are coming to be 55% of 1.2L.

Am I doing correct calculations?   Why CA is required for this?    Should I ignore builder's letter for VAT?