Here is excerpts of rules of cancellation:For Tatkal Tickets Booked as e-Tickets:
A flat refund of 25% of total fare charged on ticket, excluding Tatkal Charges is granted on cancellation of confirmed Tatkal tickets, which are cancelled up to 24 hrs. before the schedule departure of train. No refund on confirmed Tatkal tickets when cancelled within 24 hrs.of the schedule departure of train. For contingent cancellation and waitlisted Tatkal ticket cancellations, charges will be deducted as per existing Railway rules.
source: //www.irctc.co.in/beta_htmls/etktcanc.html