Most probably don’t. You dont need to be that precise, what is the point, what does it achieve?
Just ride around threshold and let the terrain take you over and under. Or play games… do a soft attack, settle back in as hard as your dare, put a hard attack in for various durations settle back in at the limit you think you can hold for the duration of the interval. That is good enough, has the desired phyisological response and replicates real life riding.