The goal of the 1RM is to determine the maximum amount of weight that can be lifted one time (i.e., 1-Rep-Max (1RM)). It is important to avoid fatigue during the warmup by performing to many repetitions so finding a suitable starting weight is something to keep in mind.
How to:
A warmup should consist of a single set at a weight that allows five to ten repetitions.
Based on initial efforts, determine a suitable weight for a second set that allows for 3-5 repetitions and then rest for 1-minute
Determine a suitable second set that allows for 3-5 sets repetitions then rest for 2min. In general, increasing the load from here in increments of 5-10% should be the safest approach,
Performing a single set of 2-3 reps then rest for 2-4 minutes, following this set should be the maximal load.
Based on the prior set the 1RM should be within range, attempt a heavier load (5% increase) and if unsuccessful take another rest period (2-4min) and decrease the load by 2.5%, if successful increase the load by 2.5-5%.