Each time you throw an Eye of Ender, it flies toward the nearest stronghold before falling to the ground. Its flight path forms a straight line on the map. Throw a second eye from another point to get a second line — the stronghold is where they intersect. That's exactly what the Stronghold Finder calculates for you.
Measure the direction you're looking, not the flight path: Aim the crosshair directly at the floating eye, then read the value after “Facing” in F3—the first number in the second set of parentheses. 0° faces south (+Z), 90° west (-X), 180° north (-Z), and -90° east (+X). Press F3+C to copy your position and viewing direction as a command to the clipboard. You can paste that line directly above.
Ideally, throw the eyes from two points at least 500 blocks apart and offset sideways from the direction of flight. The larger the angle between the two lines, the smaller the error—the tool shows you the angle and the rounding error of the F3 display.