I have not used an umbrella reflector with a hotshoe flash. I have used umbrellas indoors with studio type strobes. My strobes have a socket so that the rod of the umbrella goes directly through the reflector of the strobe. Thus, the flash is aimed straight into the center of the umbrella.
I want to use a flash and umbrella outdoors which requires me to use a battery operated flash. I don't want to go to the expense of buying a studio type strobe capable of using a battery. This is especially true since I have several radio controlled hotshoe type strobes.
My question is since the hotshoe flash mounts higher than the center of the umbrella (red arrow) the flash will hit the umbrella off center (blue arrow)...
Would this offset cause any problems with uneven lighting? Or am I worrying about nothing of any importance?
I guess that I could use a softbox. I have one which I use with a hotshoe flash quite often... I shot this image with the hotshoe flash mounted on a camera bracket and modified with a rather small softbox...