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Define the endpoint value
Let B be the positive initial investment value, E the ending asset value excluding the separately counted distribution, and I an income receipt at the end. The formula assumes the same currency and that E and I cover the stated investment boundary.
Add the income once
Terminal wealth is E+I under this treatment. Divide its gain above B by B. If E already includes the distribution as account cash, use the full ending account value without adding I again.
B is positive; I is a separate end-period income amount.
Do not silently reinvest earlier income
An earlier distribution retained as cash contributes its observed final cash amount. If it is reinvested, its ending value can differ. The period-end formula does not supply a reinvestment rate for an earlier receipt. State the actual income treatment.
Use another method when owners change capital
A deposit or withdrawal during the period changes the invested base. Subtracting it from the numerator alone may ignore how long it was exposed to returns. Exact subperiod linking or a stated money-weighted calculation answers that different measurement problem.