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I think it's basically to rate yourself: 0% is a complete failure, 100% is complete success. I guess it's a point system behind: if you get n of n points, you have 100%, whatever the n points are.
You can inspect (for geeks only) the tutorial files, e.g. Resources/tutorials/tutorials_Seattle/mission_output/ga_landing_tut/ga_landing_tut.txt: For this examples it seems to have four points, each counting as 25%:
I hope LR will not prosecute me for unhiding this
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