I fall into the camp that thinks if you're building a wheel for money you absolutely should be using a tensiometer to set a reasonably precise average tension. That said, with the relatively wide range of reasonably performing tension values on a conventionally spoked wheel, Musson's general advise to tension until the (appropriately lubricated) nipples become somewhat difficult to turn will probably put you in the ballpark. Do absolutely use plucked spoke tone to make sure that tension is even, however--if you just true to a reasonable dimensional tolerance you can end up with pretty wildly uneven tension. Aside from that, a tensiometer is the only reasonable way to be precise about absolute tension.