SharePoint Developer

I am using a SharePoint List for data entry in a hospital facility. I need to make a SharePoint List that will auto-populate a column based on the previous entry of a different column in the same list. For example, to record water usage from a meter, I need to subtract the previous Item entry from the Current Item entry to calculate total water use. I am not able to figure this out so I am looking for a developer to create this feature for me.