Subtracting from different cells

Hello,

I am tryig to figure out how to make F2 the total tach time. The tach started at 58.6 (C2), and everyday the tach time goes up, so it's an ever changing cell to subtract from. How do I make F2 the total tach time? F2 should read 22.9 right now... (basically SUM(F5:F17) ) but the -81.5 always changes...

Thanks in advance!

Screen Shot 2014-04-22 at 3.46.20 PM.jpg

Okay here is the spreadsheet: 528MW.xlsx

Picture:

Hi and welcome to the forum

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