This is why they invented flash meters--to introduce another variable and earn some money out of your pocket. In the end, your guess is about as good as anybody else's. Then, that's why they invented digital cameras, so you could shot it, look on the back of your camera while everybody in the picture was still there, and either delete it or shoot another. When it's all done and you get a "good print" you see for yourself bounce light off a ceiling is a bad idea. A better idea is to drag your shutter as much as you can hand-hold, and bring up the direct flash as little as you can get away with. Generally for that, the little dial on the back of the flash works out pretty well, on the Japanese flashes I've ever used. Sunpak 611 user for 37 years speaking here.