Hello,
See sir there is no formula to calculate the alimony, it all depends on the claim made by her in the petition
She will not have any right on your ancestral property but she might make a claim on your self acquired property if substantial time of marriage is concluded.
As of now you may go ahead and purchase the property on your own.
Your salary, expenditure and saving will play a major role in deciding monthly maintenance and not alimony.
Regards