I should also add that often after you land that first fish, you will find that the half hitches have moved about a millimetre. If you haven't tightened up the FG properly in the first place, this can move by two or three millimetres. If this happens, I always just push the half hitches back down to the base of the knot. A FG knot works fine even without the half hitches but I have found that the knot seems to be a little more resistant to shock loads with the half hitches. Don't ask me why - I don't understand this.
Finally, if ever in doubt with your knot, please cut it off and tie a new one. Losing a good fish is a hell of a price for a knot failure.