giannisg2004 is right, but this isn't just a film photography dilemma. Balancing ambient light and flash is tricky with film and digital. Newer TTL flash systems make it easier to achieve a balance but you really need to understand the underlying principles at work. A good resource althought very digitally biased is strobist.com, everything there can be applied to film photography. You can use a dslr along side your EM to speed up the learning curve, also as previously mentioned a flash meter is a valuable tool for this. I use a Sekonic L-558 if I have no DSLR with me. One last thing, a higher sync speed is going to help immensely... if you are doing this type of photography a lot or if your are not completely attached to the EM.. consider a FE2 or similar (with a 1/250 sync speed).