My question is, how income tax is calculated for partial redemption.
Same as normal. The redemption should always be treated as FIFO. Say you are buying 10 units every month [I know the units maybe in fraction and price would be different every month and you are investing fixed amount].
After say 9 months you have 90 units. Now when you sell say 45 units, you are actually selling 10 units from first 4 months and 5 units from 5th month.
So calculate the price at which you purchased these units. This becomes your cost.
Now when you sell, you know the price. So subtract the sell price from cost price. This is your taxable income.
Short term capital gains is taxed as per your tax bracket. So add this taxable income to your other income and calculate taxes accordingly. You have to pay tax in advance and not wait till year end. You can do this online as well.