Would you like the same discount rule triggered on the presence and quantity of a particular item, or would you like the discount to only take into consideration the one item and discount only that one item?
If you wanted to just update the trigger for this discount to be based on the quantity of a particular item then you can just adjust the current trigger you have to make it based on the 'Quantity of a specific item in the cart' and specify the id of the item. The rest of the discount should be the same.
If you want to have this only trigger on the item and apply only to that items subtotal you will need to also adjust the calculation portion. You will need to set it to 'Based on an item's column value', then specify the item id and choose the total price column.