The extra interest is only useful when we have a considerable amount of satoshis left in our freebitco.in account.
But besides that, more number of FUN tokens directly gives us more WOF spins which directly gives us more rewards.
So my aim is mostly towards accumulating more rewards.
I recently withdraw a nice amount but before that I had about 0.1 BTC on freebitco.in so the extra interest you got, thanks to the premium membership, was a nice amount and it added up pretty quickly. As you mention, you also get more daily WOF spins which is why I think 12 500 tokens is the sweet spot.
Current interest rate is 4.08% and with 10% interest boost by holding 12.5k FUN tokens the total interest rate becomes 4.48%.
By holding 0.1BTC you were getting around 0.00001315 BTC per day which I think is pretty decent.
I hope my calculation is not wrong here. Let me know in case.
Your calculations are rigt. It might seem as a small amount daily but after some time it is a decent sum and is also one of the main reasons I wanted to have at least 12500 tokens. With the interest I have right now a little bit above this tresholf and I will probably sell these when we get a big pump in the prize.