In an MSU, aside from SIO/SIF octets there are also other octets such as flag, crc, etc. The total other octets aside from SIO/SIF is always fixed at 6. The use of this factor is case to case basis. It depends on how your switch measures the SS7 Traffic. If the element gives you total octets then there is no need to use this factor but if its counting only the SIO/SIF octes then you need to add 6 octets per MSU to get the total octets. There is also a case that the measured octets is of SIF only, then you should use 7 instead of six.
The formula written below by Nick is correct. The use of 64000 instead of 65536 is also correct.