Screenwriting uses a lot of very specific terminology: inciting incident, lock-in, obligatory scene, and planting & payoff... to name a few. But when it comes to the five major plot points, writers often have a hard time understanding and establishing the difference between the first culmination and the main culmination.
The First Culmination is your midpoint, and the best way to set it up is by KNOWING YOUR ENDING. To know your ending is crucial to the structural blueprint of your script because A LOT of decisions are determined in direct response from the ending. For example, if you know your hero wins in the end (achieves his/her goal), then the first culmination is usually a high point for him/her. But if it's a tragic end, then he/she should experience a low point at the first culmination. Basically, the ending and the first culmination mirror each other. Both highs or both lows.
The Main Culmination (a.k.a. the End of Act Two) is the highest obstacle for the main character(s) in Act Two, and in most every script it is at a stark polar opposite to the first culmination. After the lock-in completes Act One, you have spent four sequences (usually between 45 -60 pages depending on the genre) with your character(s) struggling through the main tension of the story while trying to obtain his/her/their objective. The main culmination is the build-up and end result of all of that. It is the greatest obstacle, the last alternative, the highest or lowest moment as our main tension comes to an end, giving us the first inklings of a new tension that will carry us through Act Three.
But remember, the eight sequence approach creates a mirror for the midpoint & final resolution: both high points or both low points, so the main culmination goes the other direction. If your hero wins at the midpoint AND at the end of the script, then he/she should have his/her lowest point here.