Hello,
TurboTax also have the feature to auto calculate the other fields for you if you just provide the input for the dividend. If you enter the value in box 24 there and move forward to the next step, it will populate other fields (box 25). However, the reason why we allow input in the other fields is to provide a way to override the values in case the value in the payslips is different from what is auto-calculated.
Regarding your question on catching any human input errors during the inputs, TurboTax already have the auto calculation feature which use the correct formula to calculate the values for you. If you choose to override the value, then there's no validation on that again
