You worked prior to 15.12.1995. Also you worked after 15.12.1995. To you the pension calculation formula is different.
Following formula is applicable to you:
Past Service benefit (plus) Pensionable service benefit (less) Proportionate reduction subject to minimum pension specified for each group.
What you need to do is as under:
1. For past service benefit you have to calculate pension amount for the period beforeáthe 16/11/1995.
2. For pensionable service benefit you have to calculate the pension for the period after 16/11/1995 as per the formula mentioned by you.
3. You need to check the proportionate reduction subject to minimum period depending on when the pension started orádate of commencement of pension.
Calculation of pension amount for the period beforeáthe 16/11/1995 is very complex subject which one need to study. I had given presentations on this calculation but right now I have forgotten it. Till last year I had the presentation material which I discarded it in 5S carried out by me.
12th June 2018 From India, Mumbai