So I've been shooting in M a lot lately and really getting goodie results. (sharp and nicely exposed photos). I have has some issues with deterring proper exposure using the meter while using a flash. For example, I was shooting my son opening some presents yesterday at his bday party and the meter shows a couple increments underexposed when activated, but when I shoot photo its properly exposed due to external flash. My question is how so I determine proper exposure prior to the shot with an external flash? I know there is a pilot button that fires a quick burst but I'm not sure that is enough time to determine proper exposure. Is there a technique to this?