C++11 variadic templates calling a function inside a class

I'm learning variadic templates in C++11. How can I call test.finder as a functional argument of test.var_finder ?

#include <functional>
#include <iostream>

class c_test {
    public:

        double finder(double a, double b = 0) {
            return a + b;
        };


        template<typename... Args>
        double var_finder(double c, Args... args, std::function<double (Args... args)> func) {
            return c * func(args...);
        };
};

int main () {
    c_test test;

    std::cout << test.var_finder(0.1, 2, test.finder) << std::endl;

    return 0;
}

My expected result is 0.1 * (2 + 0) = 0.2.