My rail to rail rack spacing was mandated by wind load calculations. The panel manufacturers typically show a spacing for the rails (rail to rail) measurement, to predict a certain loading ability. Look at the panel mounting instruction sheet, if your supplier didn't send one, check the panel manufacturer website.
Typically (for portrait mounting), you'd take the long dimension of the panel (64.5"), divide it by 4, leave 25% overhang off the top (16"), 50% of the length of the panel as a gap between (32"), and 25% overlap off the bottom (16"). The more you space the rails apart (rail to rail), the less wind load the panels can typically handle.
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