Unsure how to change stat use multiplier into flat increase

Basically, after doing some testing I realized the shower's upgrades increase the hygiene stat of each shower by a multiplier instead of a raw boost.  Long story short, the "Fast" upgrade gives 2x Hygiene, the Faster gives 3x hygiene, and together they give both, meaning the cheap 3 hygiene University shower skyrockets to 18 hygiene with both, and I want to nerf this. 
I tried just reducing the multiplier to 1.5 each but higher tier showers still get extremely large buffs to their hygiene rate.

I need to somehow change the "stat_use_multiplier" in this group of xml to just be a raw addition, but I am unsure how to do that, if it is even possible.

<V n="apply_new_value" t="apply_statistic_modifiers">

<U n="apply_statistic_modifiers">

<L n="autonomy_modifiers">

<U>

<L n="stat_use_multiplier">
            ^-------^
<U>

<T n="key">16657<!--motive_Hygiene--></T>

<U n="value">

<E n="apply_direction">INCREASE</E>

<T n="multiplier">1.5</T>

</U>

</U>

</L>

If it is possible or more information is needed, please tell me.