I have just tried that formula and if the *2.7 is stepped up or down to say 2.6 or 2.8 then it displays 0.00 until 2.4 which is odd to me
This would depend on the values in I15 and I25. As you suggested, commission to be calculated only if it exceeds 2.7 times. Perhaps, your input values equate to I15 being higher than I25 times 2.4.

what the 0.2 plays in the formular
0.2 or 20% is the commission rate