Nice work! I didn’t see that this existed!
I decided to test for you with the read-only API key 502066c2-fa74-4193-b269-d5d5fa68ec8a and found this:
My userscript found this:
Comparing the two, my userscript doesn’t project past level 4 for a free user, but the site doesn’t ensure that the fastest possible can actually happen (no lessons have been completed) or take into account the accelerated speed for levels 1 and 2 of 3d 2h.
I’m aware that most of these are known issues, but I thought that this could help testing. 
Despite the edge cases, I think this site has its advantages and have just linked to this thread on mine.