Hi Lucy Blue,
Welcome to the ALC's forums!
First thing I have to say is get a copy of Dr Atkins New Diet Revolution and read it. It's cheap to buy and easy to read and is key. You can do low carb without reading it but it's gonna be much,much easier if you do.
If you use Fitday.com to track your food/activities/weight etc. it will say .61g of carbs for one raw egg.
Try it out and that may help you keep more accurate track.
Normally I would only have 2 eggs and I would keep the onion slice thin (.777g carbs by Fitday.com)
Here is a link to the "Best of" Forum and an thread that is great and may help answer your question....

http://forum.lowcarber.org/showthread.php?t=15909

Again, Welcome and hope this helps!