How to get in-game variable to source

[Bisuke]

I'm planning to connect an NPC where you can set a value on a specific variable, and it will affect the formula inside the source code. How can I reference the npc variable to the formula in the src code?

 

This is the scenario

NPC will define the value of a variable, let say $WeatherBonus, the value of this will affect let say the final skill damage formula by adding $WeatherBonus value.

[Emistry]

something like this if not mistaken

int value = static_cast<int>(mapreg_readreg(add_str("$WeatherBonus")));