You have the antenna in the attic so I assume you can reach its center point. Maybe not. But if you can, just clip your vna to the feed point and take a measure. I had a 66 foot NVIS 40 meter antenna flat at 15 feet above the ground. Clipped the vna onto the wires, let hang, stepped away and observed its display with my spy glasses!!! Just kidding. Held, read it and it was spot on.
