If you're using at least 0.063" for the panel, then 0.5" spacing is a minimum I recommend from a flexibility perspective. If using thinner material, then just having a doubler, such as what Garmin provides with their G3X Touch screen install kit, should be sufficient. Understanding that you probably need at least that much distance for nut plate installation, you're probably good.
More importantly is longitudinal bracing. As long as you have a couple supports from center of the panel going forward to a larger structure, you're fine.