If you are using a flatbed scanner to get the measurements from the 21-step target, I would recommend looking at the QTR Stepwedge Tool. It is a Photoshop plugin that does a really good job of getting pretty accurate readings from the scan. Here is a direct link to it on the Roy's QTR downloads page: QTR-StepWedge-Tool.zip
You will want to get Data Tool also. It is good for smoothing and saving the data in an easy to use format. (although the latest update of my app does have its own data smoothing controls). The stepwedge tool does have its own instructions included in the zip so I didn't write additional ones for it (but I will cover it in a separate video).