I had a version built with adaptive spacing but it felt more complicated and less user friendly so I scrapped it. My main aim is to create something that teachers can and will use because at the moment the most important thing is for people to establish the routine of systematic retrieval. So, mine purely responds to successful or unsuccessful retrieval. You can manually change the spacing multiplier for yourself, I recommend an interval multiplier of between 1.5 and 2.5. Otherwise it works just like what you've described, it takes the actual elapsed interval, multiplies it by your multiplier (which is 2 as standard) and snaps it to the lesson that is closest to the ideal date (it always picks a lesson before the ideal date if possible). Any rule I think is open to being applied too stringently but generally for me anything below 80%-100% (based on difficulty of question) is just considered a failed retrieval and would require you to reset the spacing schedule.