the way that order is done now is something like this:
list:
post1 order: 1.0
post2 order: 2.0
post3 order: 3.0
...

when a reorder on post1 is done ex: is put below post2 like:
post2 order: 2.0
post1 order: ?
post3 order 3:0
the new order computed is 2.5