Database
[{rank:1,totalPoints:2000},{rank:2,totalPoints:1980},{rank:3,totalPoints:1940},{rank:4,totalPoints:1890},{rank:5,totalPoints:1830},{rank:6,totalPoints:1765}]
Query
db.collection.aggregate([{$project:{diff:{$abs:{$subtract:[1800,"$totalPoints"]}},doc:"$$ROOT"}},{$sort:{diff:1}},{$limit:1},{$project:{_id:0,rank:"$doc.rank"}}])