5 kts below bug speed ? Quit trying to calculate spool up time and pay attention to attitude and power. If you leave the speed there , it will only decay more as pitch is maintained / increased to maintain approach profile. Careful with pitch trim as a remedy here too. If you have to calculate spool up time in this scenario , you are reaching for the wrong remedy. Hands on the thrust levers , scan and aim for what is your normal thrust setting for the approach. Getting below bug speed requires significantly more thrust to stop the trend , requiring further pitch and power changes. Best to be anywhere else but below bug speed for the obvious reasons. Pay attention at all times. Finally engine spool up time relates to thrust lever position. If it is at idle it takes time... while you are counting....everything else goes to s**t. Good luck.