When using the 'decode outputs' on moneroexplorer for a TX that I sent, I get this warning when it (correctly) estimates the amount that I spent:
Possible spending is: spent amount (tx fee included)
Note: without private spendkey, it is impossible to know whether this is your real spending.
So do not take this number seriously. It is probably totally wrong anyway.
What are the potential inaccuracies of simply assuming that the spent amount is equal to the sum of the user's inputs minus the sum of the user's outputs? Additionally, how could the private spendkey be used to accurately determine the amount spent?