While I may have a goal in mind, I use my average time to perform the task as the basis for my par time. I begin with 10 repetitions with the timer as a start signal but no par and move to a par time set for .2 slower than my average time. I then reduce the par time by .1 every 10 repetitions until it is .1 faster than my average time for the task.
For example, if I had a reload that averaged 2.0, I would do:
10 reps with no par (focus on clean technique)
10 reps with a 2.2 par
10 reps with a 2.1 par
10 reps with a 2.0 par
10 reps with a 1.9 par
Fifty reps total. As my average time moves down, I adjust the base par time accordingly. So, if I started hitting my reloads consistently at 1.9, I would start setting my par at 2.1 and finish at 1.8.