Zen Cart Version 1.3.0.1 patched

I am using the Zone Rates as the mode of shipping.

I have set the max package weight as 20kgs in the admin. When an order of 3 items is placed, with the weight of each individual item as 9kgs resulting in a total order weight of 27kgs. the estimator correctly splits this into 2 packages but then proceeds to also split the total weight in half, or 13.5kgs.

This should be one package of 2 items @18kgs and another with one item @ 9kgs.

Is this a function that I can edit to correct? or one that for the time being must be accepted?

A pointer to where this math is processed would be helpful