1. 더러운 코드

private void InitDefaultItems() // 줄여서 Init으로 쓴 이유가 무엇일까
{
    if (!ExistDefaultItems(Data.Income)) { // 뜻은 통하지만 Is나 Has로 시작하는 개선의 여지가 보이는 작명 
        LoadIncome();

        var lastOtherIncomeItems = ConvertStringToOtherItems(otherIncomeItems); //String은 파라미터 형식으로 유추가능
        Income.AddRange(lastOtherIncomeItems);
    }
    Income.EnsureMinimumItems(() => new OtherItem(), MinItemCount);

    if (!ExistDefaultItems(Data.Expense)) {
        LoadExpense();

        var lastOtherExpense = ConvertStringToOtherItems(otherExpenseItems);
        if (lastOtherExpense.Count > 0) {
            Expense.AddRange(lastOtherExpense);
        }
    }
    Expense.EnsureMinimumItems(() => new OtherItem(), MinItemCount);
}

// Income과 Expense을 다른 구문으로 나눠서 적용해도 좋을 듯

1. 깨끗한 코드

private void AccountBtnClick(AccountInfo account, bool isIncome = true, bool isNew = false)
{ // Button이 길어서 Btn으로 줄여쓴걸까.. 하는 역할에 따라 이름을 적절히 변경 가능해 보인다.
    var window = Cursors.Wait.ShowCursor(() => AccountWindow.Load(account.Name, account.Id, new OwnerlessWindowPosition(this)));
    bool isExistDuplicated = account.GetDuplicatedCount(isIncome ? Income.Select(x => x.Account) : Expense.Select(x => x.Account)) > 1;
    string originalAccountName = account.Name;

    window.ShowOwnerDialog(() => {
        if (isNew && isExistDuplicated && MessageDialog.Show(Messages.Get("Do you apply account to all?"), window, MessageDialogType.Question, "Apply Once;Apply to all") == "Apply to all") {
            ApplyAccountsId(originalAccountName, window.AccountName, window.AccountId); // 왜 Accounts?
        }
        else if (isIncome) {
            Income[idcIncome.FocusedItemIndex].Account.Name = window.AccountName;
            Income[idcIncome.FocusedItemIndex].Account.Id = window.AccountId;
        }
        else {
            Expense[idcExpense.FocusedItemIndex].Account.Name = window.AccountName;
            Expense[idcExpense.FocusedItemIndex].Account.Id = window.AccountId;
        }
    });
}

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
    if (values[1] is TimeUnitType timeType && values[2] is Laytime laytime && values[3] is bool naLaytimeCalculator) {
        if (naLaytimeCalculator) {
            return null;
        }

        return laytime.IsOperation ? GetLaytimePortDaysString(laytime.Operation.Actual.Total.RemainedDay, laytime.Operation.Actual.LaytimeAmount) : GetLaytimePortDaysString(laytime.Estimation.Laytime, laytime.Estimation.DemDes);
    }

    return null;

    string GetLaytimePortDaysString(double remainDay, double demdesPrice)
    {
        string days = timeType == TimeUnitType.Days ? "days" : "hours";
        remainDay *= (timeType == TimeUnitType.Days ? 1 : 24);
        string saveLost = remainDay > 0 ? "Laytime Saved" : "Laytime Lost";
        string demDes = remainDay > 0 ? "Despatch" : "Demurrage";

        return $"{saveLost} {Math.Abs(remainDay).ToString("N6")} {days}  {demDes} {Math.Abs(demdesPrice).ToString("N2")}";
    }
}