获取最后阅读数据并添加到下一行Cake PHP

Sorry if this is a simple one but I have spent days on it!

I have a system built in CakePHP version 2.2.4 that allows a user to input a meter reading via a simple form. The form asks for:

1. Reading Date
2. Previous Reading
3. Current Reading
4. Average flow (Worked out by: Current Reading - Previous Reading = x divisible by total days since last reading = Average Flow

I am trying to get the /add.ctp to automatically pull in the previous reading and work out the Average flow as above.

I need as much help as you can possibly give guys!