How to calculate rate and hourly/daily averages?

Hello,

Can you please help me with the correct way to enter a formula to calculate rate in Zoho Reports? Example: I have a totalizing meter taking readings every 15 minutes. I am wanting to create a rate column that takes the difference of two readings over that time period (15min). Is there a way to do this?

Also, is there a formula I can use to get hourly and daily averages of that 15 min data?

Thank you for the help.