I'm learning a new language, Turkish, and I've tried to follow as much as possible Duo's advices. Here is my tree with 34 crowns:
1st line done : 5
2nd line done : 5 5
3rd line towards level 5: 5 4+4
4th line towards level 4: 3+2 3+2
5th line towards level 3: 2+2
6th line towards level 2: 1+1
7th line towards level 1: 1
When all objectives are completed, I increase the target level for every line, and so on, so my new goals will be :
4th: 5 5
5th: 4
6th: 3
7th: 2
8th: 1
Then I'll start again doing one lesson per unit on all lines plus the next :
4th: 4+1 4+1
5th: 3+1
6th: 2+1
7th: 1+1
8th: 0+1
Etc...
The advantage is that it's not too repetitive, it's the perfect opposite of doing a whole unit to level 5 or 6, spending hours and days on the same topics, same words. It's the concept of spaced repetitions. When I'm done with lesson 1 of line 8, I'll go back to line 4, in that case it will be 6 lessons later (depending on your rythm it could be 1 to 6 days later, and note that this tree is not very wide, if you have many lines with 2 or 3 topics, it will take much longer).
As far as I have understood, Duo will lay the path according to this progression. And if you don't follow it, Duo will probably compute a point in the path. Before that point you'll find units you'll have done, and after this point you'll find a mix of units done and units not learned. So after you have finished a unit, Duo will unlock one or more units depending on your history.
This is of course only a theory, but it seems plausible to me and I'm pretty sure that if you follow a "standard" progression you won't be bothered by the path change.
PS: I still have an hesitation regarding checkpoint 1. Should I push all topics to level 5 or continue as if there was no checkpoint?
My current favorite option is to make all skills to level 5 and then do all legendary levels before going past the checkpoint. This would be like a kind of end of year examination.