A ReactiveUI question about summing properties from a collection based on a filter.

Program.cs

using DynamicData;
using DynamicData.Binding;
using ReactiveUI;
using ReactiveUI.Fody.Helpers;
using System.Collections.ObjectModel;
using System.Diagnostics;

namespace ConsoleApp1;
/** Simplified for this question, but let's say I'm building a UI with this layout
 * | Name    | Should be Included| Size |
 * | Record 1| [ ]               |5     |
 * | Record 2| [ ]               |5     |
 * ... etc.
 * 
 * Total: the sum of items' size where their "Should be included" checkbox is true
 */


// For some boilerplate code here we go

// Our "Record" which has the properties from the table above
public class Record : ReactiveObject
{
    public string Id { get; set; } = string.Empty;
    public string Name { get; set; } = string.Empty;

    [Reactive]
    public int Size { get; set; } = 0;

    [Reactive]
    public bool ShouldBeIncluded { get; set; } = false;

    public Record(string id, string name, int size)
    {
        Id = id;
        Name = name;
        Size = size;
    }
}

// A "View Model" that can be used to display them
public class Test
{
    public ObservableCollection<Record> records;

    // How do I keep Total up to date with the correct value, in this case 15?

    [Reactive]
    public int Total => records.Where(r => r.ShouldBeIncluded).Select(r => r.Size).Sum();

    public Test()
    {
        records = new ObservableCollection<Record>();

        //records.ToObservableChangeSet(x => x.Id).Filter(x=> x.ShouldBeIncluded). What's next? Is this right?;
    }

}
public class Program
{
    public static void Main(string[] args)
    {
        // Create a new test object
        var t = new Test();

        // Add 10 records to it, each with a size of 5
        for (var i = 0; i < 10; i++)
        {
            t.records.Add(new Record(i.ToString(), $"Record {i}", 5));
        }

        // Mark 3 records as needing to be "Included"
        t.records[0].ShouldBeIncluded = true;
        t.records[3].ShouldBeIncluded = true;
        t.records[5].ShouldBeIncluded = true;

        Debug.Assert(t.Total == 15);

        t.records[6].ShouldBeIncluded = true;

        Debug.Assert(t.Total == 20);
    }
}