Exemplo

double global = 0.5;

class MyClass
{
public:
int useGlobal(int input){
int result = (input -30) * global;
return result;
}
};

Solução

Encapsule as variáveis na classe.

class MyClass
{
public:
int getMember(){
return member;
}

void setMember(int input){
member = input;
}

int useMember(int input){
int result = (input -30) * getMember();
return result;
}

private:
int member;
};