Line of Sight

Effective LoS = Recon LoS×2 ＋ Radar LoS ＋ √(Fleet total LoS - Recon LoS - Radar LoS). This is the formula used since Spring 2014, 2-5 boss node and any node with the seaplane-searching animation.
ONLY Recon planes' (yellow & light green planes WITHOUT A LOGO, see this) LOS values are doubled. LOS from attack planes (red & blue) are not doubled, but in fact square rooted. Do not double your attack planes' LOS value when making LOS calculation.
You know you fulfilled the LoS requirement if you see a short animation with a seaplane flying to the node and back to your ship icon, plus the flagship's image appearing on-screen. Otherwise, you will be diverted off-course without the animation playing.

5:06-5:11 of this video here shows what the animation looks like: http://www.youtube.com/watch?v=J-x1tNuCeSE

You can have a high total LoS and still FAIL the LoS requirement. Do not use the total LoS as a guide.
If you have trouble reaching LoS requirements, take a look at other players' reports (link is underneath every event map as <Report your results Here> heading on this page) and adjust your equipment accordingly.
There are many player reports where players of similar HQ levels have differing effective LoS requirements. This hints to a possibility that effective LoS scales to level. Do not be surprised if you level up or reach the next HQ bracket (e.g. HQ lvl 60-99) and fail to reach boss node when you just reached boss node the previous sortie.