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Tying a string to a direct manual flash is an old trick, since the fall off it predictable. It works quite well, especially when lighting groups with a single centered light; just arrange the group in the arc the end of the string traces and lighting will be even across the group.
Back in the 70s I used a variation on that method using a single power Graflex flash and shooting for standard paced-off distances of 16, 11, 8 and 6 ft. closing the lens a stop with each closer distance.
But with the instant feedback of digital I find it easier for the model to simply hold up a gray card (for checking WB during editing) and white towel (for exposure) and fire off a few test shots until the towel is 1/3 stop under clipping in the playback. That takes ambient overlap into account. When using dual flash and wanting a full range of detail I'll add a black towel target into the mix to gauge the fill level, again adjusting the overlapping key flash based on the white target clipping. Get the shadows and highlights rendered correctly then everything in-between will be also.