Thanks a lot for taking the time to check this, that's very useful info.

Although the reason I haven't set 20% as the default, is because I'm concerned this may create too many situations where guessing would be required. So I'd need to have the solver in place. So thank you for the link as well.

Although now, I'm wondering if the solver can be smart enough to allow situations like this, where finding how to progress isn't quite obvious, like this one: https://news.ycombinator.com/item?id=39234686