m1280m,
If you have a column with the time or date corresponding to each data entry, then you can easily search the time / date column to find the maximum value (i.e. the last data entry), and then use a VLOOKUP function to find the corresponding data point.
E.g. If your dates are in Column A, and your data is in Column B, the following should work:
=VLOOKUP(MAX(A1:A100),A1:B100,2,FALSE)
Using this method, your data entry points don't even need to be in date-sequential order - the function will find the data point corresponding to the latest time / date. The "FALSE" flag is required to find an exact match on the time or date - if you omit this term, the function may return an incorrect value based on an approximate match. (I have assumed 100 rows is more than enough to accommodate all of your projected data needs - increase the range to something bigger if required.)
Alternatively, a workable "kludge" is to set your "reference cell" to point to A30 (as in your example). When you need to add some more data, INSERT a new row above Row 30, copy the old data from what is now A31 into the inserted A30, and enter the new data into A31. The formula in your "reference cell" will have updated to point to A31. Clumsy, I know, but it might help.