Have you considered putting the critical items into an Anki deck and using the Anki app on your smartphone to revise them?
- You have to have the Anki app installed on your smartphone (or computer). The Android version (Ankidroid) is free. The ios version costs about US$40 because they were made by different people.
- Go to WaniKani to Anki Exporter web site.
- Copy/paste your API key, which you can find in your PROFILE / SETTINGS page.
- You can adjust the percentage to 70 by clicking on “Additional Options”.
- Click on the “download” button below “Critical Items”.
- Follow the instructions in the DOCS link.