How to determine the correct hardness setting for Water-Right Sanitizer Plus Softener
Ed Beal is right about personal preference. Although water that is too hard or too soft can also have implications for your water system and equipment such as pumps, dishwashers, etc.
Dealing strictly with the issue that your wife is having with soap not rinsing off easily you'll need to increase the hardness of your water. In order to determine how much, go to a local pool supply store and purchase a simple pool/spa water testing kit. You don't need anything fancy, you only want to measure PH levels. Follow the instructions and measure the current PH level of your water. That will give you a base point. Slowly adjust your water softener to increase the PH in small increments. Once you have it at a comfortable point measure it again and use that as your target PH level. You should be able to find info on adjusting PH in the instruction manual or on-line.
The link you posted in your question explains how to adjust hardness when following the installer setup instructions.