You might also try using an interval/countdown timer.
Your last D1 was 23 rounds. Suppose you want to beat that by 1. Set your timer to countdown 20 minutes / 24 = 50 seconds. Do a set every 50 seconds.
I switch back and forth between using an interval timer and just doing it by feel. I sometimes think I get more sets by going all out for as long as possible and then slowing down when I have to, than I can get by trying to follow an even interval. But I got my PR on L4 using the timer.
Just remember, you're only going to work this hard for 20 minutes, so go all out. You can chill when you're done.