I nearly finished the first draft of Act Two last night. At this point, the script is 77 pages long. Technically, according to basic structure 101, I am 13 pages short of where I should be.
If you look at introductory guides to screenwriting, the standard structure is as follows: Act One - 30 pages, Act Two - 60 pages (pages 30-90), Act Three - 30 pages (pages 90-120). Of course, this only follows if you're writing a 120 page script. Though 120 is "standard," the trend I've seen is to cut that down closer to 105. For action and horror, that's about what many writers target; anywhere from 90 to 105 is decent length for those genres. Romantic comedies and dramas can be a bit longer. It has been a long time since I've written a 120 page script, and even longer since I set out to do so.
So how do you readjust? An easy way is simply to do it proportionately. For example, if you know you're going to write roughly a 100 pages script, or are targeting that as a final page count, just reduce everything page-wise, keeping the ratio the same. Acts one and three are 25% of the script; act two is 50%. Therefore, you wind up with 25 page acts one and three and a 50 page act two. By this math, assuming I'm targeting 100 pages, I am two pages over where I should be. In the grand scheme of things, that's not bad, especially for a first draft. In reality, I'm actually aiming for a 95 page script - maybe even 90. So, the other solution to maintaining act structure for a shorter script is one I employ a lot: use the structure, until it begins to use you.
Structure should be malleable. Yes, there is a definite, agreed upon industry standard format out there. Know it. Even if you don't use it. But that format is more a guideline and an unbreakable set of rules. If your second act is five pages short, so be it. Any reader, producer, or agent worth their salt will be able to tell when something is stretched or gratuitous. And you know what? That will be the first thing to wind up on the cutting room floor. So screw it. You don't need it, and it doesn't need you.
I also don't follow the structure exactly. I'll aim for the "appropriate" 25 or 30 page first act depending on the final page count, but my third act rarely tops 20 pages these days. It's often closer to 15. I believe that if the first two acts have been successfully written, the ending will come quickly and practically write itself in the process. By the time I'm hitting the climactic final sequences, everything is coming to a head and simply can't sustain much more. There's no room for extra dialogue or exposition or action at that point - it's all been set up, and the proverbial volcano is erupting. No keeping it from overflowing any more.
Granted, I am focusing on action movies now. If you watch some, you'll probably notice that the last 15 or 20 minutes comprise the final showdown. Everything else has been taken care of or lost by then. That's how I like to structure my scripts, too. Dramas and comedies might be slightly different.
The bottom line: act structure is a great guide, but it's one that is best tailored to the needs of your script.