I am sorry to sound harsh but you really should not be advising clients on such matters without a firm understanding of these issues,  The HSE do publish information on Incidence and Frequency rates calculation but severity rate is used outside of UK and in some major UK  companies.  You need to get to grips with the pros and cons of each and the problems with their use and calculation.

Unless of course this is a construction client with a "safety advisor" who thinks they are clever.

You are pretty close with the construction client safety advisor. I am fairly comfortable with the incidence and frequency rates, it is the severity rate that has thrown me a little as its not something that we are usually asked for. Thanks