Applying quantity discount automatically based on Qty entered in "Add to Cart"?

I'm using Zen Cart 1.3.8a -

I'm stocking items that need to be priced differently depending on the quantity being ordered, i.e.

1-2 = £17.40
3-4 = £16.35
5-10 = £15.50

I want to customers to be able to enter the quantity of items they want in the "Add to Cart" box (e.g.. 4) and then the total cost be figured out automatically (e.g. 4 x 16.35 = 65.40)

I have so far only been able to use an option name set to "drop-down" and then setting up approriate values - but obviously this requires the customer to choose from the drop down which quantity range they are ordering and then entering the quantity of items they actually want in the Add to Cart box.

I'm sure I'm probably missing something incredibly obvious - hopefully someone can steer me in the right direction.

I wouldn't exactly call it "obvious"...

Admin > Catalog > Product Price Manager > select category and product, click Display.
Below the "Preview Only ... Current Price Status ... Preview Only", click the edit button.

At the bottom of the next screen you will see a button
Add 5 Blank Qty Discounts:
[Add Blank Discounts] click this.
This will show a table with

Product Discount Info . Discount Type: . Discount Priced from:

Select the type of discount and enter qty/price numbers.