Rounding to the nearest Value Based on Values from another table.

Solution Excel @ https://1drv.ms/x/s!Akd5y6ruJhvhuWkS2pLB0BHnAigv?e=DDSrQv 

Use this for Table 1. See the working here - Open a blank query - Home - Advanced Editor - Remove everything from there and paste the below code to test (later on when you use the query on your dataset, you will have to change the source appropriately. If you have columns other than these, then delete Changed type step and do a Changed type for complete table from UI again)

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WclTSUTJSitWJVnICsozBLGeQmCWY6QJkGpqCma4gUQMw0w3ENAcz3UEKDMFMDyATIugJZAH1xwIA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Location = _t, Value = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Location", type text}, {"Value", Int64.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Low", each Table.Last(Table.SelectRows(Table2, (x)=> x[Column1]<=[Value]))[Column1]),
    #"Added Custom1" = Table.AddColumn(#"Added Custom", "High", each Table.First(Table.SelectRows(Table2, (x)=> x[Column1]>=[Value]))[Column1]),
    #"Added Custom2" = Table.AddColumn(#"Added Custom1", "Result", each if Number.Abs([Value]-[Low])>=Number.Abs([Value]-[High]) then [High] else [Low]),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom2",{"Low", "High"})
in
    #"Removed Columns"

 If you need test code for Table2

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlCK1YlWMgWThhCOEZSCCBoDebEA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", Int64.Type}})
in
    #"Changed Type"