Since it is possible to vary the number of sentences per round, it would arguably make more sense to have the “Max reviews per round” setting as a percentage than a number. This is especially relevant when playing in “infinite” mode.
I blame infinite mode and a fixed max-reviews number for now having tens of thousands of review sentences in my backlog. 
is that how it works now, where 50 is the denominator?
@morbrorper I manage this by setting the maximum reviews per round to zero. I play new sentences separately to reviews.
The thing is, I like to have a mix of new sentences and old ones for review.
Bumping this thread, I would like to request this feature too!
As the number of new sentences I play in a session always depends on how much free time I have at hand, so it’s always not the same number.
It would be great if the option can be in % instead of absolute numbers.
I don’t mind it as an option, but I’d rather keep the option of a fixed number of sentences as well. I HAD reduced my FFT reviews to under 9,000, though I note that this morning they’re up to 9,059 again, presumably from questions that I did months ago, sigh 
. FFT is not something I do any longer (I may go back to it when I run out of other things), but I still want to clear that stack for the sake of neatness. I generally do that 50 or 100 questions at a time, depending on how much time I have available. A percentage would not be meaningful for me there, so I’d rather retain the option of a fixed number.
(When I review FFT I set my 100% mastered review time to “Never” which SEEMS to avoid the problem with “Ignoring” them which I discussed here. The jucier ones which I do still need to review I add into my “by subject” collections.)
If we were to have the option of percentages, to my mind it would make the change to the “sentences to go” count discussed here more important so that you’d have some idea of how far you have to go. If you chose “100%” then after the start you’d need to keep count manually of how many there are to go.
I would like to bump this thread again, since I was just about to post a similar request: I have the same issue as @versuss, in that I like to have a mix of reviews and new sentences, but I never know how many sentences I will be playing.