Curious about what percentage you guys use when doing a single set, to failure or not, especially for squats.
By percentage do you mean of one-rep max? Since I have not done single rep max lifts in decades, I can't comment. Perhaps others will. But if you are alluding to how many reps people do for their sets, in my own case it varies from exercise to exercise.
Specifically for squats, you my not be interested in my numbers because I do weighted ATG pistol squats. I do what I believe are referred to as "breathing squats." So if you can do, say, about 10 squats at a given weight, you try to go for about double that. As you approach your limit doing normal squats, you start resting longer at the top of the movement and breathing more between the remainder of the reps until you reach your absolute limit. In my case, towards the end of the set I'm taking 3 breaths between reps. Admittedly, it is not in the true HIT style, because, while catching my breath at the top of the movement, my knee is locked so the tension is not continuous. But I'm still standing on the working leg only, holding the weights, and with my other (non-working) leg off the ground in front of me. Presently, I do between 17 and 19 reps for each leg with the weights I'm presently using. If possible, I plan to increase the reps to 20 in due course rather than increase the weight. I take a 3-minute rest between my right leg and my left leg to catch my breath and properly recover. By the time I finish my left leg, it takes me a few minutes just to get my breathing back to normal.
Upper body rep ranges are lower, from 8 to about 16, depending on the exercise, and all are done with continuous tension.
I hope other people chime in with their numbers.