mike mills
Not sure if this will help or not...

A1 would be the 1021 or whatever the ititial reading is
A2 would be the first new reading of 1345
B2 would be = (A2–A1) Copy down (Fill) the formula for all of column B
For days missed row 3, 4, and 5 copy down the last value (fill) which is 1345 (or retype the new number in each block)
New data on day five 1657
You can use the Average command in Column B, in this case 127.20 if you wish.

Chart would look like this..
1021 First Data
1345.0 -------324.00
1345.0 ----------0.00
1345.0 ----------0.00
1345.0 ----------0.00
1657 ---------312.00
-------------- 127.20 Average