Yes, you cannot the predict the profits for entire year. You might have a great profit in April-June Quarter, but you cannot extrapolate the profit to say that you will earn at the same rate until the end of the financial year. It could be more or less,
Best way to pay advance tax payments are by paying tax for that particular time period.
Eg : For Sep 15th - calculate and pay advance tax for what was earned till then and same applies for Dec 15th. For last quarter i.e 15th March close to the year end - you can make all the balance payments as you would have fair idea on how you will end the year.
You can claim refund of excess paid advance tax, if any. Tax refunds are processed quickly these days by the IT department.
If you make 50Lacs profit April to June quarter - you need to pay 15% of tax applicable on 50Lacs profit earned before 15th June.