Turns out that the walls are (of course) not exactly square and now I have a gap in between two sheets. They touch at the wall side and the gap grows to just under half an inch at the other side.
Using my laser level I put a pencil line where it would be 100% straight and aligned, lengthwise.
If I keep going like this, all my sheets will be off. Especially because I have to stagger them. What’s the best approach?
So far I only have these two sheets up and I'm thinking to either:
1. Cut off a 0-3/8” strip with my Dremel oscillating tool and drywall blade, I can make pretty straight cuts with it. That way I can get the next sheets aligned way better. The gap between the butt joints will still be there and need to be fixed by the drywall finisher.
2. Replace the last full 8x4 sheet with a new one or re-attach the current one. The butt joints will be better, but new gaps will arise on the wall side. This will be hopefully covered up by the wall sheets.
3. Basically start over and replace or re-attach both sheets. Not sure if I can re-use the sheets or cause a lot of work for the drywall finisher.
Speaking as a drywall finisher, this gap is not a bit deal; one swipe of quickset mud and it's gone. The question is how to proceed: of you can find two adjacent walls that are square, follow those. If no walls are square, strike a chalk line square to the wall you want to work from, and that will give you a measure for how much to leave on one end of your board or the other. Remember: if you're going to drywall the walls as well, you can have up to a 1/2 inch gap on the edges that will be covered by the wall sheets.
I should add: even though this space is no big deal, the sides of your sheets are no longer straight, so, like you said, it will create a similar space moving forward unless you take the one sheet down and reinstall. Again, just strike a chalk line and line the side of your sheets up with that.
By the way, your screw countersink is a beautiful thing. The stuff I've seen homeowners do...
I went ahead with option one, put a chalk line up and made the cut. It's not the greatest cut and I still have to touch up, but it's now straight (see red dashed line) and most of my other sheets are now installed as well. All straight and I'm happy that I did this.
@Nathanael: thank you for your advice and even more thanks for your compliment. That was really awesome to read. Since the start I've been kinda nervous about the screw depth since I had never worked with drywall before. I bought a special drywall bit that automatically pushes out when the screw has reached the ideal dept. It was $20 (Canadian) well spent.