You have to know the number of bytes sent in each direction of the CCS7 (uplink / downlink) during the call set up. Usually the values are different for a Mobile Originated Call and for a Mobile Terminated Call, so it would be fine if you had this values:
BHCA = MOC + MTC
… and the number of bytes associated with MOC and MTC for uplink and downlink.
I know these values for GSM, but I cannot help you for CDMA. Just to put an example, in GSM you need to send about 200 bytes in the uplink (CCS7 link) to set up a MOC.
Then take the worst case (usually downlink).
You also need the load of the SS7 link (0.2?, 0.4?, 0.5?). Usually