hmm..a small correction they are called H&D curves. But more importantly I wish someone would explain to me how they can calculate a 30 minute exposure from an H&D curve unless they made a family of curves specifically for long exposures.
An H&D curve is used to represent tonal and exposure relationships at NORMAL times and from what I have studied and read it is impossible to correlate a timed exposure this long to a published H&D curve made with normal times. Perhaps he was talking about development of the film, given an exposure which records the minimal detail it is possible to "guesstimate" the development time.
I could be wrong, but as you have explained it, I dont think it is possible to do this.