I believe in the past (unless my thoughts are off at the moment) it is as you say 30 secs is not only 15 in and 15 out- but also a steady flow. ie empty to 1/4 to 1/2/ to 3/4s to full should be steady.. (not just 15secs overall but where the first part is fast then it slows in the middle and the last fliing is fast- ie uneven…
so say “1 minute breath” (milestone) but instead 64seconds (64 vs 60secs) .. 64 secs would be 32in, 32 out, which is 8secs to fourth, then 8 (16secs) to half.. etc.. and do 3 or 4 breaths in a row. ie doing a single breath and then having to change your breathing rate to “rebalance” relax tension built-up and pant.. (I only mention all the painstaking detail above, from my work on it- trying to do a 20 sec breath, 10 in and 10 out.. smooth and steady is tricky but then if you find you need to “pull out of the practice” it is still a burdening (O2 deficit and most of all internal releases needed.. tension in body but also ‘tension’ in mind… the fluids flow differently
I hope that helps (I felt very wordy as I wrote this, so it came out as quite long) cheers