When electing to track an Action with Milestones, begin with four by default and automatically have the percentage entered (25%, 50%, 75%, and 100%)

Ideally milestones would automatically calculate % per milestone the same way the checklist works (allowing overrides, of course). Having four by default is also smart because this is one of the easiest ways to coach/lead the end user in defining the path to completion. Here are some example "placeholder texts" for the default milestones:

25% - [Describe what a quarter of the way done looks like]
50% - [Describe what halfway done looks like]
75% - [Describe what three quarters of the way done looks like]
100% - [Describe the final milestone to complete!]
You can't beat tracking milestones directly in-line on the planner. This is meant to help incentivize/speed up TTV with that lovely workflow! per ALS Association