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I've used one flash in a few 7 ft umbrellas. I have a couple other larger flash options Ike a Profoto 600R, an Einstein and VML and am on the list for a B1 but some times you just don't have the time to set up or have that stuff with. Keep the umbrella close to the subject, shoot on higher power and if possible try shooting through if you have one that's translucent. You can get it even closer without sticking your talent with the umbrella shaft. I think the 7 ft translucent is a great product. I use it outside as a scrim, a diffuser for shooting thru. If you can find one one more flash unit, borrow or buy, it obviously works better.
Go for it though. It's how we learn.