I know its very difficult to quantify western blots accurately. From what I know, the best way is to expose your gel to equipment which picks up the signal directly.
This method is the best for quantifying western blots however it is more time and obviously requires the right equipment, which is sometimes hard to find or expensive to buy.
If you cant get a hold of this equipment, then I suggest to try very quick exposures.
Even an instant pressing down on the cassette and taking it off quickly may get you some decent results.
How long are you exposing now in your cassette-film?