I openly explain to the players that I was facing the other way when the ball was played, and therefore by the time I've formed a picture players are in a different place too when the ball was played.
Can't use that excuse when following play and facing the right way, but that is when a wide angle is vital. Not at all suggesting you need to be out by the touchline, but equally you won't get it right if you run up and down the middle of the pitch as you can't possibly have players on both touchlines in view.
In my experience as an observer, where I do have a side on view always, referees on average get around 60-70% of offside decisions correct when on their own.