[{total_project_cost:1000,projects:[{_id:1,name:"Proj 001",cost:25},{_id:2,name:"Proj 002",cost:550},{_id:3,name:"Proj 003",cost:425}]}]

db.collection.aggregate([{$project:{projects:{$map:{input:"$projects",in:{$mergeObjects:["$$this",{percentage_revenue:{"$divide":["$$this.cost","$total_project_cost"]}}]}}}}}])