Perhaps different Ministries/statboards have different processes, but my understanding is that the formula they use will include a number of factors such as years of relevant experience (not so much age), sector, education level etc. It does not depend on your last drawn salary because the formula they use is to ensure some level of parity within officers in the civil service.

For example, I got a pay cut (monthly gross) when I joined the civil service. When I asked why, I was cited the reasons above.