Back during the infancy of this software, most people would use it for just two reasons: to store data (especially customer database) and as a glorified calculator. Perhaps because those are a spreadsheets intended or more practical uses or that people are not really that trained to use its features to the fullest. You would even be surprised that up to now there are office employees who, when they are prompted to make some calculations, they would open their spreadsheet software and do the calculations there. However, as weird as that may sound, it is one of the ways that a spreadsheet can help speed things up in the office. Especially if the employee has been working on spreadsheets for a long time that making fast calculations using the cells has become second nature to him. But there are other good uses for this application.
The next thing to look at, what are the goals of the client? Many clients like personal trainers simply to help them work out, no specific goals but general overall fitness. For these clients, percentages are probably unnecessary as you as the trainer can monitor their intensity and effort by watching. If it is an athlete preparing for their sports, intensities will give you a great guideline to go by and allow you to accurately pick a weight for their work sets that will increase their potential for strength.