I honestly think this should be the standard for all binding referendums with answers "keep it as it is" / "change it". Maybe not 75% but at least 55%. Implementing a change at 50% is so ridiculous. It can swing around by a single news story, weather at the polling date etc.
When we take them back there needs to be clear repercussions for leaving though. We need to disincentivise going back and forth about it.
As a starter any trade or economic agreements made, that are touching any aspect that is normally negotiated by the EU as a block need to be revoked in their entirety. Also i think some form of compensation for the damages caused to the EU by brexit should be paid.
Otherwise going back and forth will be a wedge happily driven by Russia, China and others.
I mean, because it would benefit both the EU and the UK in many ways. Not to say there aren't downsides to letting them rejoin, but there are also significant upsides that we really shouldn't ignore.