It's definitely best to have a tight fit, but having a small gap at the bottom I have found does not make much difference. You will lose a few bees (a few dozen a year) that crawl under the FB and then can't find their way back under and die. The bigger the gap, the more bees lost, until it gets big enough, at which point it's not very useful as a follower.
I try to get mine to fit within 1/8".