if you have bar lights, the helmet light doesn't have to be too bright.
I just strap on a small 450lumen nightrider or cygolite. Their strap mountings make it easy to mount to my helmet - and they are darn light.

off road I might do a 900 lumen on my helmet, but it has these drawbacks
- significantly heavier
- causes glare with road signs (esp if wearing glasses)(not a problem in the woods, but can be on the road)
- will blind oncoming riders as it shines at their face.