Not everyone needs to get to 90, but you can measure to get an idea.
Measure:
1. Distance from center of jaws to cab
2. Distance from center of jaws to edge of bed (90 degrees to first measurement)
Best way to do it is measure back from center of pin a distance equal to measurement 2, then measure the width of the fiver from center of trailer(pin) to edge at that distance back from center of pin. If this is greater than 1, you'll hit. If this is close to 1, you'll probably hit unless you are always on a level surface. The tilt of a trailer through a turn can lean it into the truck a few inches.
That will tell you where you are at for sure, but still isn't a guarantee, lots of variables going on. Every trailer is different, every person is different, everyone's camping is different, and everyone's attention while backing is different.
I'm not telling you to buy a slider. I've told you my reason for choosing one and that I don't regret my decision, nor would do it differently. There are plenty of people that tell you NOT to buy a slider because they don't have one and are ok. What support will you get from them when you take their advice and damage occurs to truck? It will probably go like this "huh, I've never had that problem". Then you go through the hassle of fixing your truck and paying for it.
People do well with them and without them. There are plenty that buy manual sliders and rarely or never slide them, and plenty that didn't get a slider (or didn't use the one they had) and busted out a window and dented a cab corner. An automatic slider is the only one to make sure you don't have contact at anything less that full on jack knife. Everything else is a calculated risk.
You need to consider the risks and make
your own decision based on what
you'll be comfortable with. I knew I'd have a lapse in judgement or attention and have an issue, so I bought the hitch that guaranteed it wouldn't happen.
On edit: I'd suggest reading through this thread and tallying up the busted rear windows:
Click
