Comment by gwd

This is one of the reasons I ended up writing my own language study system. I haven't used the FSRS thing mentioned above, but I've heard it's much better, and having looked at the algorithm, it seems like it should be better.

The other reason I wrote my own system was to integrate SRS with extensive reading. Basically, my algorithm tracks the difficulty of all the words and grammar concepts, like FSRS; but then it gives you content at the right level for learning (either fewer than 5 new concepts, or an average of 95% known material).

And among the things that fits, it balances reviewing older material and learning newer material, based on what would have the largest impact. (Reviewing something you're about to forget has a bigger impact than learning something new, because the new thing you're going to forget much more quickly. So the balance of new / review and spaced repetition falls out naturally.)