It all depends on the configuration of your trailer.
If you don't have an offset pinbox, then go to one of the front corners of the trailer and (assuming you have a curved front cap - see my avatar for an example) drop a plumb line from the corner portion of the cap that extends farthest forward. Measure horizontally from the center of the trailer's kingpin to the plumb line - that's what you'll have to clear, so I'd add 2"-3" minimum to that measurement.
If you have an offset pinbox that places the kingpin ahead of the front cap, you will normally have to clear 1/2 the width of the trailer at a 90* angle, so I'd take 1/2 the trailer's width and add the 2"-3" minimum to that measurement. For our 102" wide 5th wheel, that would be 51" + 3" = 54".
If you want to clear past 90*, that's a different kettle of fish. I'm assuming you don't.
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