Hi Chris, I will try to add a few more perspectives about how I am interpreting your need (in addition to the guidance that Howard has provided.)
If you happen to include the grade in the student's grade book by importing these external results, wouldn't the student know that they have completed the activity by seeing a grade in their grade book?
Well, if you really want student's to "check off" something, yes, you can still do this in Moodle. Enable "Activity Completion", then create a "Label" resource, and turn on activity completion for this label. Then the students can check it off. I mocked this up in my attached graphic.
If you find yourself doing this for a lot of items, you might want to take a look at the "checklist" plugin. I've been using this plugin along with activity completion. Some students like one tool, others the other tool. In my screenshot, I am showing you part of this checklist plugin, as I see it as the administrator.
Relative to Moodle's grade book and weighting, Moodle's grade book is powerful and can handle almost anything that you want to throw at it. But, it does take some time to master it. The grade book can be simple or complex. The best way to explore this is to first, grab a cup of coffee (or favorite beverage), relax, and spend some time with the grade book documentation and experimenting. Do a lot of testing, if grades are important. Put some "bogus" students in your course, give them grades on items, and see if the grade book is producing your desired results. I can't stress enough that this will take some time, and don't let it frustrate you, learn.
It is always difficult to communicate one's exact needs and desires about grading. You might try mocking this up in a spreadsheet program like Excel, and then asking "How would you do this in Moodle?" Many years ago I believed that my grading scheme could only be done in Excel. But then I spent time with the Moodle grade book and have not used Excel for grading for 10 years.