The following statement, or similar statements expressing the same intent, shall be included in definite quantity goods or services contracts:
"Variation in quantity. Upon the agreement of the parties, the quantity of goods or services or both specified in this contract may be increased by a maximum of ten per cent provided (1) the unit prices will remain the same except for any price adjustments otherwise applicable and (2) the procurement officer makes a written determination that such an increase will either be more economical than awarding another contract or that it would not be practical to award, .another contract."
Haw. Code R. § 3-125-8