Looking for a formula for "Given $x_1$, $\ldots$, $x_{n-1}$, the output $F(x_1,\ldots,x_n)$ is dependent of $x_n$ and is of $l$-bits security"
To be well defined as a function, you need to specify lengths of the $x_i$, which is not the same thing as their entropy. Also, the $H(h_i)$ in your equation should probably be $H(x_i).$

Let's assume each $x_i$ are binary vectors of length $n$ for concreteness. Then what you want in a more general version of this problem would be
$$
\mathbb{H}(F(x_1,\ldots,x_n) | x_1,\ldots,x_{i-1},x_{i+1},\ldots,x_n) \geq \mathbb{H}(x_i) \quad \forall i=1,\ldots,n
$$
which also takes care of the unequal entropy case for the different $x_i.$