Zakaat on retirement annuities

Asaalamu alaikum WRB.Question is regards Retirement Annuities.I understand RA is permissable.How to calculate Zakaat on the RA.If I have had the RA for say 10 years and have been paying a monthly sum of say R 300.In past I have been taking the sum paid by me and paying zakaat on that amount. ie : after ten years I would have paid R 36000 in instalments — so I paid R900 zakaat on the 10th year.
Was this correct.