how can we calculate the percentage of the missing values to determine if it less than 5%

Hi
you said that there is Rule of thumb : if you removing less than 5% from observation ,feel free to remove all missing value
I calculate the percentage of the missing values using the formula data.isnull().sum()/len(data)*100 
and I got 3.9% for price
and 3.4% for EngineV
and I added them 3.4+3.9 =7.3% >5%
so why did we delete the missing values when their percentage is more than 5% of the observation?