I don't own a flash meter, so I wing my exposures, starting with the highest sync speed (1/180 sec) and bracketing apertures. I need the greatest possible depth of field for the billowing smoke to look sharp. At ISO 100, with the flash heads a stop or so down from maximum, my aperture is usually f/9. If the smoke seems soft, I crank up the heads and go to a higher f-stop, increasing the ISO setting if there isn't enough light to get adequate depth of field. I rely on histograms and try to place the peak toward the right of the axis, without losing detail. Since smoke is almost entirely highlight, any clipping of the histogram spells failure.