I think Chiffon means, to compare the variable rate to the fixed rate you can get now. For example, 2.89x5=14.45, if the prime increases in the 2nd yr, you have a space of 2.89-2.4=0.49 for 4 years, so you are indifferent from variable or fixed if the prime increase by 0.6+0.49/4=0.7, otherwise you should pick fixed.