Testing Flash Power ??

[Couldbe]

I have loaned my flash meter to a friend, who is now having trouble locating it. So I will have to use my camera's hisogram to determine proper exposure.Can someone provide the proper method to test flash power. I have 4 different brands with different manufacture power rating to test.I have heard use a gray card and some say a white towel at 10 feet.
Thanks,
Mike

[zona5101]

if I'm not using a meter, i usually set my lighting by just chimping. (looking at the lcd image). I'll turn on highlight warning to check for blown out highlights rather than look at the histrogram. YMMV.

[Couldbe]

Thanks for the reply and I agree with your statements. I'm trying to determine the actual GN for the flashes, not expose a scene properly. In a dimly lit room, I will try to get a 18% gray card exposed properly with the histogram spiked in the middle and then I will multiply aperture x distance to get the actual GN. I think this will work, but also heard to soot a white towel and expose just still clipping the highlights, then do the math.
Thanks,
Mike

[Dajuan]

Yeah, the scenario of grey card and histogram spiking in the middle would provide an accurate reflective reading.