Sir, right you are being wrong at the first place.
In Tatkal also, if one remains Waitlisted, the deduction is only Rs. 20 and not some 20 + Tatkal charges.
If it gets confirmed and one wishes to cancel, he looses the 100% Tatkal premium and 75 % of the General Quota cost.
What you received 85 on you Tatkal cancellation is correct and...
more... I assued, you ticket costing would have been appx. 340 + Tatkal charges.
Thanks!