I am trying to do some template customisations and show a subtotal amount for all items 'from a category'

1)

I have added a Dropdown custom category field within items

2) 

I added a drop down custom field with

Category Name 1

Category Name 2

Category Name 3

3)

Within my Estimate Template I can show this Category Name by using the placeholder

${ESTIMATE.Item.CF.Category.Value}

4)

I created new subtotal fields for my category names which go into my template with placeholders between the 

standard ones..

%EstimateSubTotal%

Add up all item amounts for Category Name 1 and show that value in

${estimate.cf_subtotal_of_category_name_1}

Add up all item amounts for Category Name 2 and show that value in  ${estimate.cf_subtotal_of_category_name_2}

Add up all item amounts for Category Name 3 and show that value in  ${estimate.cf_subtotal_of_category_name_3}