Brokers providing fair margins

Is there any broker, preferably an online broker who charges margins exactly according to NSE rules, especially in case of F&O. e.g. If I do a covered call position with NIFTY future and option then broker should charge me

SPAN + Exposure (3% + 3%) - premium

However most brokers seem to be adding arbitrary additional charges especially for spread positions.

Also on a related matter, is there any portfolio margining benefit for exposure margin? SPAN takes into account portfolio effect for computing margins.