I use an abacus
Seriously, it takes practice. With expierence you kinda just know how far to lead. The hurricane is a good plane to use to practice deflection shots thanks to all that tracer.
Another tip is not to get too close behind your target if you only have wing mounted guns. You don't want your bullets passing the target before they have converged on your crosshairs. I forget the exact distance that the convergance is set at in this game though. I'm sure someone else knows.