The problem with what you are trying to do, is to get an actual average you need to keep track of how many values are added up to be averaged.
for example, you have a 5 and a 10 for an average of 7.5. if your next entry is another 10 your average should be 8.33, however if you don't keep track of how many items you have and take the average from R4 and add the new value and then average you end up with 8.75. I will think on this some more, because we could have S4 be a running total of all the values, and T4 be a running total of how many entries. then R4 could be an acurate average.
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