Weighted loot decision from a pre selected pool

Hey guys. I am doing a xml based loot table system for my framework im making . I am trying to do weighted loot drops using these tables.

I basically have a list of items correctly loading and spawning in the crates except i beleive i am not doing truly weighted loot. Each item has a % chance between 0 and 100 which transtaltes to a max loot weight selectable by the user. Assume the max weight is 10,000 for now. so weight 0 = 0% and 10000 = 100%.

//Get references to random items
foreach (int item in lootBucket)
{
    LootItem rollItem = selectedTable.GetItem(item);

    int finalWeight = GetRandomWeight(3);

    Debug.Log("Rolling for " + rollItem.itemId + ", item weight = " + rollItem.weight + ", roll = " + finalWeight);

    if (finalWeight < rollItem.weight - weightOffset)
    {
        finalLoot.Add(rollItem);
        numberLeft--;
    }
}

This works out whether or not the random Weight is less than the weight of that of the object. if it is it gets spawned. But i am under the assumption that truly weighted loot uses the sum of all weights in the list of loot. How would this be applied to my method?

Adding to my reply from RobAnthem this is what i have got

//Caclulates the weights by iterating through the list, adding each weight to the end range of the previous + 1.
public void CalculateWeights()
{
if(items.Count > 0)
{
totalProbability = 0;
bool start = true;
LootItem previous = items[0];

                for (int x = 0; x < items.Count; x++)
                {
                    LootItem current = items[x];
                    if (start)
                    {
                        //First item
                        start = false;
                        current.weightRange = new Vector2(0, current.weight);
                    }
                    else
                    {
                        current.weightRange = new Vector2(previous.weightRange.y + 1, previous.weightRange.y + current.weight);
                    }


                    totalProbability += current.weight;
                    previous = current;
                }
            }
        }

//Actual selection algorithm, checks if the current item is within range and selects it based on the weight ranges set in the above function. Rater than choosing from the table of items, it uses the table as a whole and selects from that using the random weight between the total value of the tableā€¦

        List<LootItem> finalLoot = new List<LootItem>();

        if(selectedTable != null)
        {
            //Get references to random items
            while (numberToSpawn > 0)
            {
                int weight = GetRandomWeightInRange(3, 0, selectedTable.totalProbability);

                foreach (LootItem item in selectedTable.items)
                {
                    if(CheckWeight(weight, item.weightRange))
                    {
                        //We have spawned an item
                        finalLoot.Add(item);
                        numberToSpawn--;
                        break;
                    }
                }
            }
        }

        return finalLoot;