Flash Meter - the tool you didn't know you needed.
My Sekonic flash meter - A strong ally with the Force. I learned about photography back in the late 70s and early 80s. My first flash was a Metz 402 - powered by a lead-acid shoulder-carried battery pack that had enough power to light up the other end of a hockey rink. After that came a Vivitar and then a host of Nikon flashes. Then I got into strobes - big ones, complete with soft boxes and barn doors. I quickly learned that using flash didn't always result in a proper exposure. As camera sensors began to tie into flash TTL technology, the process became easier, but it wasn't consistent. Did the flash have enough power to do the job? You could play with flash output through flash exposure compensation, if your camera and/or flash had the feature. That always took playing around, and you didn't always know if the exposure was quite right until you got it back to look at the results. The problem increased significantly when not using ...