That is one of the problems with these "grow guides". (most definatelly NOT a Bible) They don't fully explain, and you can't ask questions.

The outdoor grow and street lamp analogy has been around for many years, but has little to do with us indoor growers. The sun delivers...what...a trillion lumens per square foot? (from every angle possible...reflected light) And a streetlamp at 20 feet (or so) might be a 100w per sq.ft. (numbers fudged to approximate true knowledge) That great difference is what...a couple-o-billion:1 ratio of daylight to darkness interference. Would be like a 1000w light user worrying about a candle at 100 yards.

Climb-in, shut the door, and wait a couple of minutes till your eyes adjust. If you see any light...best to fix the problem. You might be able to get away with the stress of an indicator light or small light leak once in a while, but if it's not the only source of stress, problems will likely be a guarantee.