Contract void when consideration cannot be ascertained as agreed

Where a contract provides an exclusive method by which its consideration is to be ascertained, which method is on its face impossible of execution, the entire contract is void: Provided, that where a contract provides an exclusive method by which its consideration is to be ascertained, which method appears possible on its face, but in fact is, or becomes impossible of execution, such provision only is void. R.L.1910, § 933.