Rounded Projections

How can I recieve rounded projections?

  Find<TItems>
    .Select(TProjections.ProjectionList
      .Add(Dic.Items.Tax.Id.Group)
      .Add(Linq.Subtract(Dic.Items.GrossPrice, Dic.Items.NetPrice).Sum))
    .ListValues;

I need GrossPrice and NetPrice rounded before they will be subtracted...

Well, using sql-projection would work fine, but I think "round" could be an implemented projection-function