The recommended precast panel to panel joint width for architectural projects is 1/4". It is common to have ¼” – ½” caulk joints from Window to any substrate, ¼” being most common. This is the minimum nominal joint width needed to adequately account for production and erection tolerances and still maintain an effective minimum joint width that can be caulked. A 30' long spandrel panel is allowed, per PCI tolerances up to a 1/4" variation in length. Keep in mind that of 3/8" is the minimum width that caulk suppliers will warrant. It is also important that the joint between precast panels and window frames also maintain the same nominal joint width.