You will find that there are dozens of different tutorials available online, ranging from books and courses that you can pay for to tutorials and video lessons that can be downloaded for free. Each of these courses is broken down into sections that will address each of the different sections of formula so that you can learn them in groups. This tends to make it much easier to fully grasp the necessary formula for each section before moving on to the next segment.
Nearly every strength and conditioning coach in the country relies on percentages to enforce desired intensities to athletes for certain lifts. Basically, the client would perform their lift at a percentage of their 1 repetition max. If they had a max bench press of 100lbs, and you wanted to do 3 sets of 12 reps at 60%, they would do 3x12 at 60lbs. This is absolutely necessary in most strength and conditioning environments for multiple reasons, including consistent program design as well as enforcement of intensity (speaking from experience, some people WILL slack if given the chance). So the question becomes, are percentages right for you?