Hi Steven,
Yes you would use a microflow to add up all the percents and use an exclusive split to see if the sum is less than or equal to 100.
I'm not sure how your domain model is setup, but for this example, each test is a separate record. I retrieve the list of tests and then use the aggregate list activity to sum the percents. Then I use the exclusive split to check if the sum is less than or equal to 100, and if its greater than 100, I show a pop up message.