How Do I Save Volatile Data in Excel?

I have an application that is a series of weekly spreadsheets, with data depending previous weeks' values. In simple terms think of each spreadsheet having a cell that adds 1 to last week's value. This week's spreadsheet relies on last week's data, so I must also open last week's spreadsheet. I use manual calculation (calculate sheet) in order to prevent last week's data from updating (since it depends on the previous week). So far, so good. But then when I click Save on this week's spreadsheet, it appears that Excel tries to recalculate all of the cells of all open spreadsheets, causing last week's spreadsheet value to become undefined, which then ripples down to this week's spreadsheet. I've also noticed that results seem to be better using +S vs. clicking on the Save button (which I thought were supposed to behave the same). Any ideas? Thanks!