Thanks for your reply.
expInfo[‘frameRate’] appears to be bugged - always returns 60. I used @ade_mh’s code from here: Incorrect frame rate logs on Pavlovia - #8 by ade_mh to get the actual framerates.
The reason I didn’t want to set the duration in frames is because then it would exclude anyone with monitors that aren’t 60Hz. 120, 144, even 240Hz monitors are relatively common, and I didn’t want to exclude those people from going through the study.
It’s no longer a pressing issue for me to resolve this right now, as I have decided to set the stimuli to fixed duration levels and collect their actual framerates with the code linked above, although it would still be interesting to know if frames are indeed rounded up.