In maximising one’s CPF savings, how much should the minimum amount be in one’s Ordinary and Special Accounts for total optimisation of the bonus interest and additional opportunities?

An extra 1% interest per annum is currently paid on the first $60,000 of a member’s combined balances (with up to $20,000 from OA). (Quote from CPFB)

Hence, extra interest earned is earned on both SA and MA, so as long as you have $40K combined in MA and SA, you'll get an extra 1% on them. There's no need to have $40K specifically in SA, so a 50/50 split will work (i.e. $20K each in SA/MA). The remaining $20K in OA will also earn the 1%, but it is credited to your SA.

Beyond those limits, Hariz and Larry have given some options to consider for the excess money. Usually, I don't advise my clients to invest SA, since 4% guaranteed is pretty difficult to beat. I find that I'll have to be getting at least 6-ish percent returns on my SA to have it worthwhile since I'm exposing myself to market risks and volatility.

To maximise, you should have at least $20,000 in your Ordinary account. This will give you 3.5% or $700 guaranteed.

And $40000 in your Special Account. This will give you 5% or $2000 guaranteed.

Any amount above this can then be invested if you’re willing to take some risk and grow even further. You can choose to invest in the listed products on CPFIS with some caps on stocks and gold, but as much in Unit Trust investments and Investment Linked Products.