Here the problem is not calculating brokerage, the problem is calculating the transaction value. In the example shown by me in the earlier post, the buy price is 55 paisa per lot, the sell price is 57 paisa per lot. So the buy amount should be Rs. 550 and the sell amount should be Rs. 570, total Rs.1120. Your calculator shows Rs. 1.12
Now, there are only 4 currency pairs traded. We don't know whether the options will be introduced for other currency pairs, if ever. At least till then, you can multiply it by 1000.
Any changes in the cost structure will need you to make changes in the brokerage calculator. The taxes could change... maybe new plans introduced from your side etc.. So, the brokerage calculator isn't a one-off thing, it will have to be kept updated with the various future changes.