This is my code. actualRevenueAmount and actualCostQty  assigned their value as actualRevenueAmount + actualCostQty . Not give correct output

ex: PMRevenueBudget.CuryAmount = 20

      PMCostBudget.CuryAmount = 10

      actualRevenueAmount = 30

      actualCostQty = 30

using PX.Data;
using PX.Data.BQL;
using PX.Data.BQL.Fluent;
using PX.Objects.CA;
using PX.Objects.CT;
using PX.Objects.PM;
using System.Linq;

namespace Lauges
{
    public class ProjectEntryExt : PXGraphExtension<ProjectEntry>
    {
        #region Event Handlers

        protected void _(Events.FieldUpdated<ContractExt.usrOnBudget> e)
        {
            var row = e.Row as Contract;


            if (row != null && (bool)e.NewValue == true)
            {
                CalculateCompletedPerc(row);

            }
        }

        private decimal CalculateCompletedPerc(Contract contract)
        {


            decimal actualRevenueAmount = PXSelect<PMRevenueBudget,
                Where<PMRevenueBudget.projectID, Equal<Current<Contract.contractID>>>>
                .Select(Base).FirstTableItems.Sum(rb => rb.CuryAmount ?? 0);


            decimal actualCostQty = PXSelect<PMCostBudget,
                Where<PMCostBudget.projectID, Equal<Current<Contract.contractID>>>>
                .Select(Base).FirstTableItems.Sum(cb => cb.CuryAmount ?? 0);


            decimal percentage = (actualCostQty / actualRevenueAmount) * 100;

            if (actualRevenueAmount != 0)
            {
                contract.GetExtension<ContractExt>().UsrCompletedPercentage = percentage;

                return percentage;
            }
            else
            {

                return 0;
            }

            Base.Caches typeof(Contract)].Update(contract);

        }

        #endregion
    }
}

@tharinduweerasooriya90  I can see two issues in your code

1. In the Field Updated Event Handler I see the first parameter is missing. It should be something like _(Events.FieldUpdated<Contract, ContractExt.usrOnBudget> e) 
2. In CalculateCompletedPerc your las line of code is never going to be executed as you are returning anyways before that point.

Also I suggest

1. move your decimal percentage = (actualCostQty / actualRevenueAmount) * 100; into the if so you won’t get error in case you have actualRevenueAmount zero.
2. assign either zero or your calculated formula to a variable and return at the end of your method
3. Use Base.Projects.Update(contract) as you already have it initiated in the Base.
4. Instead of using LINQ you can use GroupBy <AggregateTo<Sum<YourField>>> and tally your project revenue and cost.