Depending on the size and complexity of the picture, and the size that you are re-creating it, this type of project can be considerably faster than the old-fashioned oil painting or watercolor, and may be more impressive to certain groups of people. Digital art buffs are become more and more the norm in the art world, and this genre is helping to get people into something they may have otherwise ignored, all because of how it was created.
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.