The aim of sus aka System Usability scale is to measure the usability of the system:
- How ease your app is
- Dose your app require technical and prior knowledge before using it.On anothe word, measuring the learnability level.
- How ease to navigate through the app.
This test is called discounted test, it's cheap and effective. However,the best way to interpret your results involves “normalizing” the scores to produce a percentile ranking.
Now going back to your question,
- 68 would be considered above average and anything below 68 is below average, however the best way to interpret your results involves “normalizing” the scores to produce a percentile ranking.You can use measuringU tool"suscalculator" to help you with analyzing the test result and provide you with descriptive statistical view of the data.
- Whether the app was designed for external or internal use, that doesn't make a big difference as long as you have followed and conducted a proper research that follows user-centered approach. If you did so, then you shouldn't expect less result than 68.If you got below 68 then you should improve the design of the app. I advice you to prepare a proper testing plan that contains the objective of the test, the tasks the user will be doing, method you will be using such as "Thinking aloud", the Pre-and-Post-Task/Test questionnaire as well as SUS questionnaire.
I hope you find this useful.
Thank you and good luck