With the standard reflector, the Xplor 600 delivers a Guide Number of 183 in feet, or 61 in Meters, at ISO 100. So, at 10 feet, that would be about f/18 for a direct flash-to-subject distance. One takes the Guide Number, and divides that number by the flash-to-subject distance, and that simple match gives the f/number to use, at the ISO the GN was computed for, which is in this case, ISO 100. A typical bounce off of a flat, white-toned wall will reduce the GN by about 1 stop or so, give or take. Using a narrower-angle reflector, or a specialized, so-called "long throw" or "sports" reflector, can be useful when bouncing a light off of a white scrim panel, or off of a ceiling or off of a door; as is detailed in the instruction manual of older flash units like the Vivitar 285HV for example, when bouncing a flash off of a ceiling or wall, it is typically best to use a narrow-angle beam setting on the flash, and not a wide-beam spread setting. A Guide Number calculator is a wonderful tool for pre-calculating flash exposure ranges and the needed amount of flash output power, BEFORE arriving at a location. With the Xplor 600 and the standard reflector, the vendor quotes 183 at ISO 100 as the Feet GN (61 in meters). So, go to the scantips blog site and use his GN Calculator, where we see that if you boost ISO from 100 to 200 on the camera, the Xplor 600+std. reflector GN would be 289 (at ISO 200); that 1-stop ISO boost will effectively cancel out the loss of 1-stop from direct flash to a ceiling bounce; if however the ceiling bounce covers say, a fairly wide area because it's soooooooo far from the flash to the ceiling that the beam-spread is widened, you might need to go to ISO 400 on the camera, which would deliver a GN of 366 in Feet (at ISO 400). Here is a fantastic Guide Number Calculator! Understanding Guide Numbers, plus GN Calculator So, let's say you're at ISO 400 on an FX Nikon, and the flash to ceiling distance is 20 feet, and then from ceiling to bridal party is 30 feet, so in total, you've got a 50-foot throw-and-bounce...take the ISO 400 GN of 366 and divide that by 50 feet....the number is the f/stop needed, f/7.32. Maybe add a tiny bit more ISO rating to the camera and shoot .NEF mode and you'd be good at f/7.1 or f/8, and need only a tiny bit of post-processing brightening, using one Xplor 600. If you were to gang two Xplor 600's to fire as one Pop!, the GN would be 366 at ISO 200, and at ISO 400 the two-flash Pop! would give GN 518; at ISO 640 the two-flash Pop! GN would be 652; at ISO 800 the two-flash unit GN would be 732.