The point of doing a CrossFit workout is not to do the workout how it is written on the board. The purpose of CrossFit workouts is to accomplish the intended stimulus of the workout.
Stimulus means the way the workout is supposed to make you feel. When you check out any of the benchmark CrossFit workouts, you will notice they have weight loads next to them. We do the same thing at our gym - but not for the reason you may expect.
We don't write a movement, weight, or overall workout to try and see who "can do it". We write each workout with an intended stimulus in mind - an intended impact on your body.
Each week we will write some CrossFit workouts meant to take you a long time, some meant to be a middle time workout (think 12:00-15:00) and at least one meant to be a sprint. Remember that CrossFit is a general physical preparedness program which means you want to be very good at everything, and bad at absolutely nothing. That is why we do many different movements, and many different times for workouts.
To a new CrossFitter, RX'd probably looks like a random weight thrown next to a workout. I'd actually bet that even a fairly experienced CrossFit athlete doesn't really know the purpose of the weight load / movement written as RX on a workout.
RX is used to tell the athletes in a class what the intended stimulus for the day is. If you are at a CrossFit box and the coach is not telling you what you should feel in a workout, what your rep scheme should be, nor giving you any advice on what weight you should be using...please run away fast. They aren't coaching you, they are moving you through a hamster wheel of people.
To make this example, let's use the well-known CrossFit workout, Fran. Fran is 21 reps of thrusters, 21 reps of pull ups, then 15 reps of thrusters, 15 reps of pull ups, and finally 9 reps of thrusters, and 9 reps of pull ups. The RX weight is 95 pounds for men, and 65 pounds for women.
When done correctly, this workout takes between 3:00 and 6:00 to complete. Not 20:00.
As a coach, it is my job to make sure you are landing somewhere inside of that time frame. Yes, there are world class athletes that are able to do that workout in less than 2:00, and that's great. In my opinion, those athletes should be tailoring their workout to make it harder so that they fall back into the desired time frame. This means doing chest to bar pull ups instead of regular pull ups, or adding some weight.
Making sure you fall into the time frame is harder than it sounds, but it is not impossible. The best way to do this is to tell you the desired stimulus. For Fran, I would want your first set of thrusters to be done unbroken, and your first set of pull ups to be done in one or two sets. The second set of each can be broken into two sets, and the last set should again be unbroken. If you have to break up the pull ups at the end when you are exhausted, that is fine, but no more than one break.
With that sort of information, you now can start to decide what weight you should have on your bar, and whether you should do pull ups, banded pull ups, jumping pull ups, or ring rows.
This is better than giving you a percentage because 1) most people will not know their max lift for many lifts, and 2) a more experienced athlete may be able to do significantly more reps at a certain percentage than will a new athlete - so then both are instructed incorrectly when it is done by percentage.
There are many thoughts about what makes a great coach: is it their personality, knowledge of the body, or ability to motivate? In my opinion it is none of those things. All play a role, but the absolute best coaches are the ones that know how to tailor a workout for you while maintaining the desired stimulus of a workout. If you try out a CrossFit box and don't get this while completing one of their CrossFit workouts, consider checking out a different spot. It is one of the best ways to evaluate the box you're in, and the coaching you're getting.