fitzharris co sep/dec20- closing interst future rate -collar

i use the following formula to find closing interest future rate.

=closing spo ratet+(opening futre rate-current spot rate)*remaing months from borrow till expiry /total months from today til expiry

closing spot rate=current libor %+increase or decrease in int rate
opening future rate = rate at given expiry month
current spot rate= is the curent spot rate

Till now i used to get closing rate using this formula but i cam across fitzharris co sep/dec20 and i got answer different from exam kit.
for decrerease in interest rate in collar i got closing rate has 3.39 whereas kits answer was 3.9.

is this difference ok or will the answer be wrong?

if wrong, does that mean i shouldnt use this formula always?