How to Calculate the mileage difference between each invoice date for a customer

[OCBB_SFAFPandA]

Hello,

I am trying to calculate the mileage difference between each invoice for specific customers (then to ultimately avg this # out).

I was able to get the # of days between each invoice the the customers (in a different calculated column). I am having a  hard time trying to get the difference in mileage for the same time frame. 

[OCBB_SFAFPandA · Accepted solution]

Looks like I was able to figure it out. something was off when I was calculating last mileage.