I badly need a helping hand with my SD card shield

i need some helping hand about my sd card shield..

I have a project about a digital rain guage. it simply read and display the value of tips in 20x4 lcd and save the data in sd card shield.

the problem is every time the power is lost the value of tip is return to zero.

what i want to achieve is to read the last value in sd card and set this value as float variable.

something like this:

assuming this is the csv file:

let say that the last value is 12.5 before the power is lost. i want to resume in this value when the power is back.

sorry for my bad english..

tnx in advance..

start (in setup) to read the file to determine the last value written.