Research on the Drive on Sunshine feature has confirmed this was someone's theoritical design. The feature isn't even on the roadmap at Tesla.
If you want an app that can do this today, check out ChargeHQ. I found this from some old threads here and love it.
Actually, ChargeHQ has the same ability. Set your max charge level in Scheduled Charging settings
Charge HQ - Scheduled Charging Settings as the level you want from electrical (I used 20%)
...then your Charge Limit as the level you want to reach from Solar (I used 80%)
There's a 1-2-3-4 for Tesla users at
Tesla Home Charging Configuration for Charge HQ
It's exactly what Drive on Sunshine claims to be, it's just in a separate app. FWIW, the ChargeHQ app provides a nice single view that shows your Powerwall and Car on the same screen. Now that it's working, I like this better than paging around on different screens in the Tesla app.