How true or possible is this my formula, or calculation.?
Your calculations are wrong.
APY means Annual Percentage Yield and that's the interest rate you can get if you hold your fund for one year.
If you lock your FUN tokens for 90 days, the APY would be 15% and that means that you would get around 3.7% interest for the 90 days.
As hosseinimr93 wrote, and you can check this by yourself on the site:

Of course, with a longer holding period, there are more benefits as well...
