Calculate tax return
You don't need to write ...ifNecessary if you just describe the property you want to have in the return value, and then you just know that after calling it, it has that property. You want the return amount to be clamped so it is less than or equal to the maximum amount. It doesn't matter that it might not change it, but it DOES matter to the business logic that it has been applied (i.e. checked). The actual name isn't that important in this case because I think I'd drop it anyway, as per below.
First, I just rewrote your code to use a member variable instead of being static, and without all the intermediate variables:
public class TaxReturn {

    private static final float MAXIMUM_RETURN_AMOUNT= 1000f;

    private final float income;

    public TaxReturn(float income) {
        this.income = income;
    }

    public float returnAmount() {
        return clampReturnAmount(base() + addition());
    }

    private float base() {
        return income * 0.5f; // More complex calculation here
    }

    private float addition() {
        return base() * 0.3f; // More complex calculation here
    }

    private static float clampReturnAmount(float returnAmount) {
        return returnAmount &gt; MAXIMUM_RETURN_AMOUNT ? MAXIMUM_RETURN_AMOUNT
                : returnAmount;
    }

}

But this clampReturnAmount function is just Math.min by a different name. Using Math.min to enforce a limit is common enough to be well understood why we're calling it, especially if we're naming the second argument something like argument MAXIMUM_RETURN_AMOUNT. So just:
public class TaxReturn {

    private static final float MAXIMUM_RETURN_AMOUNT = 1000f;

    private final float income;

    public TaxReturn(float income) {
        this.income = income;
    }

    public float returnAmount() {
        return Math.min(base() + addition(), MAXIMUM_RETURN_AMOUNT);
    }

    private float base() {
        return income * 0.5f; // More complex calculation here
    }

    private float addition() {
        return base() * 0.3f; // More complex calculation here
    }

}