In the Dynatrace Portal, Availability is the number of successful test runs as a percentage of total test runs.
The availability chart in the Portal shows the total number of test runs and the number of failed test runs. WIth these figures, you can calculate the percentage for the successful test runs. The formula is:
(Total Test Runs - Failed Test Runs) / (Total Test Runs)
For example, if you have 600 test runs with 50 test failures, the calculation is:
(600 - 50) / 600 = 0.9167 = 91.67%