I think my problem is that I'm not using the system as intended. I learn new vocabulary mainly by reading texts or watching videos in the target language and use spaced repetition to keep track of my progress. If I can't remember a word (as indicated by SR), I'll reread the text/rewatch the video where I've first encountered it. I don't want to keep reviewing the same word in my spaced repetition program, especially not in the same session.
Interesting. I was never really happy with any spaced repetition algorithm, so I recently implemented my own dumb system which simply asks you for the number of days after which the card shall be shown again: https://github.com/kldtz/vmn
Usually my intuition about how well I know something is not too far off. If you don't specify anything, it doubles the time since the last review.