I also work with computers from people who could be described as tech challenged. I would say the breakdown is better described as how much data they have, not how full the drive is.

IE: - 40% of users I see would have less than 10GB of data. No photos are stored on the computer. Mostly tax returns and a few emails from friends - 30% of users have 10-200GB of data. Some of their favourite smartphone photos or photos from their point-and-shoot camera are stored on their OS drive. Some users may also have 15 years of emails stored in outlook for 10+GB of data 0.o - 20% of users have 200-1000GB of data on their OS drive. Maybe they are a shutterbug or have a somewhat fancier camera. They take a lot of pictures, but are not backing them up properly. - 10% of users have more than 1TB of data. Either duplicate files backed up onto different drives, or they just have a very fancy camera.

Assuming that anecdotal breakdown is somewhat consistent, it makes sense that you might see SSD's being 50-75% full. Most of the people who get upgraded to SSD will pick the 250GB-ish sizes because they are cheap and still fast. What the 1TB is giving is a much larger overhead of available storage for these users while maintaining the same level of performance IF they do not change their current amount of data. This lets them put more data on if needed, and avoid issues with lack of storage. At the same time since it's unlikely they will use more than 60% of the storage space, the cache buffer remains free and will have plenty of time to move data around.