I think 200 to 300 lumens is good for many purposes. In case you're ever caught in the dark and have to cycle through park trails to get home, it'll light up everything in front of you. 90 lumens won't cut it in that situation.

As for city night riding, it will get drivers to notice you more than just a twinkle in their eyes including not so well lit neighbourhood streets.

90 lumens are only good for campsights and home maintenance like plumbing under the sink.