I do lots of night photography and shooting in dark abandoned buildings and have my exposure times nailed down fairly well, but recently I came across a situation which I don't know how to handle:

The time is the predawn just after the first hints of brightness start turning up in the sky.

The location is a cavernous warehouse type building open at one end to the growing light.

The exposure is started when the interior is still VERY dim and extends for at least 17 minutes (I'm shooting HP5, 4x5, f16).

My question involves the mechanics of reciprocity failure. When I first open the shutter, the lighting is extraordinarily low, but by the time the last few minutes of my exposure rolls around, the light in the building must be several times as bright (still too dim to adequately meter, though). So the majority of the actual exposure probably takes place at the tail end of the period of time that the shutter is open. Is there something about the long, inadequate, exposure up until that point which desensitizes the film to that later period of brighter light? In otherwords, would the film register those last few minutes in the same way whether it was preceded by a longer period of exposure to dimmer light or not? I hope I'm making sense with this question, it just suddenly occurred to me that I might get more to register on the film by just waiting for those last 5 minutes or so rather than going for those five and the loooooong, boring 12 that preceded them.

Any ideas?