Assuming your vacuum gauge is correct, 13" sounds about right with the cam you have.

It's common for big cams to run 9" - 11" of vacuum at idle.

Spin the engine up to 1500 rpm or so in park or neutral if it's a stick car and if it's well sealed the vacuum will register 17" - 19".

The assumption is, you're reading manifold vacuum and not ported vacuum.