You need FL x 2 + 50% where FL is the lumen value of the brightest of your riding friends' lights, or you'll be riding into your own shadow – assuming, obviously, that you're quicker than everyone you ride with.
Ideally you need lights with variable settings which will enable you to up the ante once your mates apply the same formula to your lights.
Clearly you then become trapped in a lighting weapons race where eventually you can barely ride in bright sunlight because it's not bright enough.
For maximum credibility, you should build your own lights using at least ten MCEP7 LED modules along with a RadFlex controller from Clutter Electronics in Kualar Lumpar, ideally housed in a long forgotten halogen can from the likes of Loonicycle. Alternatively, pay Haddock from this forum to do it for you.
You can stop once your lights are worth more than your bike, take a deep breath and go riding 🙂