Of course you can make one unit on the NuArc equal whatever length of time you like, but I personally like to calibrate so that one second of time equals one unit of exposure. If nothing else establishing the rough equivalence makes it easier to exchange exposure data with other people.
How to do it? First, unplug the NuArc and then remove all of the screws on the top. Then, carefully lift the top and move it back out of the way. Be careful when doing this to avoid damage to the power cord of the fan.
You can adjust exposure by two different mechanisms. At the very front of the unit, in the middle, is the light sensor. You can adjust it by moving it up and down so that it sees more or less of the hole that looks down to the bulb, or you can adjust it by opening or closing the filter on the front of the sensor. If you want to adjust to one second of time equal one unit of exposure, first turn on the unit and allow the bulb to warm up for about a minute. Then open (or close) the filter (or adjust the sensor away from or toward the opening to the bulb) until time and units match. There is no point in perfect eqjuivalence so don't obsess too much about perfection in carring out this operation.