truncating decimal places

My main problem is; I need to find the last figure entered in a column and use it to make calculations in other columns.

The last figure and row is changes when new figures are added so I first need to define what cell the figure is in and then use its content to make the calculations. However the software will not allow it because I am using a figure derived from a cell that came from a formula that defined it.

Have tried all sorts of ways around it even separating the 'find the cell', placing it in a redundant cell, and then using its content in the different calculation. All are prevented one way or another.