Summing quote sub grid

I'm trying to sum a custom field in quote entity the custom field is called new_itemweight. I have tried the below code but can't seem to get my head round it. Any guidance or help would be greatly appreciated

I have added the steps in the plugin reg for Post-create, Postupdate 

My fields and entities are as follows

Parent Ent -quotes

Child - quotedetailsgrid

Column with in gris is - new_Itemweight

Field on parent for where i want the data is - new_totalweight

This is the code i'm trying to use

   public class SumWeight : IPlugin

   {

       public void Execute(IServiceProvider ServiceProvider)

       {

           IPluginExecutionContext Context = (IPluginExecutionContext)ServiceProvider.GetService(typeof(IPluginExecutionContext));

           IOrganizationServiceFactory ServiceFactory = (IOrganizationServiceFactory)ServiceProvider.GetService(typeof(IOrganizationServiceFactory));

           IOrganizationService Service = ServiceFactory.CreateOrganizationService(Context.UserId);

           if (Context.PostEntityImages.Contains("PostImage") && Context.PostEntityImages["PostImage"] is Entity)

{

               Entity Weight = (Entity)Context.InputParameters["Target"];

               var WeightTot = (EntityReference)Weight.Attributes["quotedetailsGrid"];

               decimal Total = FetchResult(WeightTot.Id, Service);

               // Updating Parent Entity

               Entity Quote = new Entity("quote");

               Quote.Id = Weight.Id;

               Quote["TestWeight"] = 10;

               Service.Update(Quote);

           }

       }

       private static decimal FetchResult(Guid quantity, IOrganizationService service)

       {

           string value_sum = @"<fetch version='1.0' output-format='xml-platform' mapping='logical' distinct='false' aggregate='true'>

<entity name='quotedetailsGrid'>

<attribute name='ItemWeight' alias='ItemWeight_sum' aggregate='sum'/>

<filter type='and'>

 <condition attribute='ItemWeight' operator='eq' value='{0}' />

 </filter>

 </entity>

 </fetch>";

           decimal TotalValue = 0;

           value_sum = string.Format(value_sum, quantity);

           EntityCollection value_sum_result = (EntityCollection)service.RetrieveMultiple(new FetchExpression(value_sum));

           foreach (var c in value_sum_result.Entities)

           {

               decimal aggregate2 = ((int)((AliasedValue)c.Attributes["ItemWeight_sum"]).Value);

               TotalValue = aggregate2;

           }

           return TotalValue;

       }

   }

}

may thanks for your help

Cheers

Dan