Zakat calculation on dormant pension

Asalaamu alaykum

I recently became aware that zakat is due on pensions and am keen to understand how to calculate my outstanding to be paid zakat in light of my personal situation.

I had a private pension that I paid into for a few years from around 1999 onwards after which I stopped. I don’t have any historical statements except a recent statement showing the current value of the fund from the company that now handles my pension after the original company was bought out.

How do I go about calculating (or estimating) the zakat I should have been paying?

Would really appreciate any help or advice.

Wasalam

May Allah bless you.
My opinion is that if one did not know that zakat was due on the pension in the past, then he/she do not have to work out the missed zakat payment. At the most, he/she could just work out the current value of the pot and pay a 2.5% one of payment to cover all the previous years.
Of course going forward he/she will calculate the zakat on the current pension fund every year.

And Allah knows best!