Tax on property sale (under construction)

Hi, I have bought an under construction property. I have been paying to the builder since lasst 7 years in installments. However, now I would be selling this property without registration and of course without occupation to someone else. The property will be ready in other 6 months.
My question is:
How do I calculate the LTCG on this? As I said I have been paying to the builder since 2011-12 and the last disbursement was in 2017-18. The new buyer is required to my last 2 installments.
So I actually have both LTCG for the disbursements made before last 3 years and few short term gain for the disbursements made after the last 3 years. So should I first calculate the LTCG with indexation and then the short term capital gain. In both the cases I will have to calculate for each disbursement to the builder as the timeline is spread over.
Can you please help me verify if this is the right thing to do? Appreciate it so much.
Thank you,
Pradeep