Use the flash on full power, and take meter readings from the middle and ends of the group and average out the reading, and try to bracket, remember that it is the flash that sets the exposure so use the fastest speed possible, it will make no difference to the final exposure,with the flash mentioned, 40cl4 you really should not have any problems, and you could depend on the flash meter built into the flash unit, If you have a wide angle diffuser then fire the flash using that until the flash itself gives you a good exposure reading and use that.