keeping the value of a cell constant

Hey

Context - I'm designing some KPIs, and I've already looked at pasting special but that would mean that the user would have to actually do stuff and I'm trying to make it as automated as possible.
So basically what I'm attempting to do is make a running account of data for 52 weeks of the year, and have it plotted on a graph. It all works fine as long as the data is inputted chronologically but if the user decides to go back a few weeks because they forgot to add some data in, the data for the following weeks that had already been put in disappears.

I'm using

=IF($D$8=Q18,$D$13*100,IF($D$8>Q18,$D$13,"stick"))

so I'm comparing D8 with the Q column, putting the percentage (D13*100) in, and if D8 doesn't equal Q18 anymore I just want the value that was previously in that cell to stay there, rather than changing

Does this make sense? :P

Thanks in advance for any help.

I did find this
http://excelribbon.tips.net/T008264_..._Is_False.html
which kind of helps with the logic but I'm trying to avoid macros!