I don't have the tool to open your sample qvw at the moment 😔,  but  the looks of your Excel I suggest that you have trouble getting the correct average due to one row with no value.

Try these expressions

• avg(Rental)/sum(total Rental)
• avg(total Rental)/sum(total Rental)
• (avg(Rental)/sum(total Rental))-avg(total Rental)/sum(total Rental)