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.
Well, let‘s take a look at your situation. What level are your athletes or clients at? New athletes or clients probably would not benefit from percentages as much as more advanced clients, as they need to learn how to perform the lift correctly first and foremost. Second, a new client will adapt to the exercise so fast that by the next time you perform the exercise, their 1 rep max may be out of date and thus their percentages would be incorrect (if they used to bench 100lbs, but after 2 workouts they can bench 110lbs, that 60% you have them working out as now too low to cause the adaptation you are looking for).