Are you going for the layered tapered look for you or your client? You could choose to measure the exact same way as above and you will be much more in control of where to cut in the layers. If you want to be frugal, you may be able to go one length shorter by measuring from the middle attachment/row and down to the longest length to achieve what you want to accomplish.
If you are purchasing the hair for yourself, make sure to coordinate with your hair professional before placing your order. Accordingly, you are assured that what you want to accomplish is possible with what you are purchasing.
Measure from the attachment point highest up going down to where you want the hair to end, and determine what needs to be trimmed or cut off. This measurement will indicate what length you would need if you wanted an even hair length without any layers.