It is common practice for personal trainers and strength coaches to use the percentage of 1RM to prescribe load of for sets and reps. The problem with this is the amount of work an individual can do at a given percent of 1RM varies...greatly.
In order to counteract this, an autoregulation system such as RPE or VBT in order to gauge effort. This will allow you to program according to the individual needs of your client as well as prescribe the proper load to use for whatever quality you are trying to improve.
My preferred method is RPE (rating of perceived exertion) because it has been the easiest to implement with clients. Although VBT velocity based training has a lot of interesting literature supporting it, so this may be something to visit in the near future. In the meantime, while this SIP continues, I have been really enjoying seeing how I can apply the science of training with the use of minimal equipment. I plan to do a post on that soon. Stay tuned.