Sum Function Like Excel Where Specific Cells Can Be Chosen

We have multiple groups that make up a large number of Capital Projects including the actual spend of those projects. We have also been given a large sum of money to make those projects happen. We would like to be able to subtract actual costs from each group from the the total sum of money given to show how much money we have left. There are also times we need to be able to do math on specific cells and have to export everything to excel, having a sum function that works like excel (where cells can be chosen) would be extremely beneficial.

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