The employer and employee may agree in writing to withhold an amount greater than, but not less than, the amount required under section 235-61, HRS and this section, and as calculated in the tables or formula method provided by the department. Any additional amount withheld shall be considered tax required to be deducted and withheld under section 235-61, HRS, and this section.
Haw. Code R. § 18-235-61-07