Fixed weight progression is more about long term progression. With less than 12 weeks or even 16 weeks to devote to the method it would be best to use a different option. Which might not make it ideal for a 6-8 weeks gen prep phase.
Furthermore, Crossfit preparation is a complex animal; you have to be strong and efficient in so many movements that focusing for a long time on only 3-4 exercises might not be the best long term option.
The approach I'm using at the moment with several of my crossfit athletes (including 2 who are going to the Games) is a continuous ramp approach.
Basically you pick 3 exercises that are variations of one movement pattern. For example...
Sumo high pull
You organize from weakest to strongest.
Then you ramp up the weight. You start off with the weakest lift and gradually ramp up until you hit the max weight you can lift for the selected number of reps. When you reach that point you continue to add weight but switch to the next movement.
Set 1: overhead squat 135lbs x 3
Set 2: overhead squat 155lbs x 3
Set 3: overhead squat 175lbs x 3
Set 4: overhead squat 195lbs x 3
Set 5: overhead squat 215lbs x 3
Set 6: front squat 225lbs x 3
Set 7: front squat 245lbs x 3
Set 8: front squat 265lbs x 3
Set 9: front squat 285lbs x 3
Set 10: back squat 305lbs x 3
Set 11: back squat 325lvs x 3
Set 12: back squat 345lbs x 3
Normally you want 3-4 work sets per movement, but at the beginning of thbe ramp you do 2-3 lighter sets of the first movement. Plan the jumps so that you can get 3-4 work sets in. Some people might not have huge differences in some lifts and the just must be smaller to get the volume in.
We use a 5 weeks block... 5rm the first week, 4rm the second, 3rm the third, etc.