find last 2 entry values for calculations

Simple question; after a lot of searching I eventually discovered a way of finding the cell id and value of the last entry. However it appears that trying to (in this case) subtract the value of another cell, all with one formula, fails and I would like to try to find out why.

If anyone can help it would be much appreciated.

It seems it is necessary to create 1 or 2 separate cells containing the values and use those to find the answer.