Read the angle from F3: the "Facing" line shows two numbers in parentheses - use the first one (yaw), with its decimals, while your crosshair sits on the floating eye.
Every thrown eye of ender flies toward the nearest stronghold, so one throw gives you a direction and two throws from different spots give you an intersection - the stronghold sits where the two lines cross. That is all this calculator does, with the same math speedrunners use: your yaw angle from F3 becomes a direction vector, and the two rays are solved for their crossing point. The wider the angle between your two throws, the sharper the fix, which is why you walk sideways (not forward) between throws.
Eyes point at the stronghold's starting staircase, not the portal room, so treat the result as the place to start digging and caving. Travel smart: convert the estimate with the nether portal calculator and cover the distance at 8:1 through the Nether, then surface and throw a confirmation eye. Once inside, the portal room is usually within a few dozen blocks of the staircase.
Stand still, throw an eye of ender, put your crosshair exactly on the floating eye and read the second number of the "Facing" line in the F3 debug screen - that is your yaw angle. Note your X, Z and that angle, walk a few hundred blocks to the side, and repeat. The calculator intersects the two lines and gives the stronghold's coordinates.
If you take the second throw from (almost) the same spot, or you walked along the eye's flight line, the two lines are nearly parallel and there is no clean intersection. Move sideways - perpendicular to the first throw's direction - at least 100 blocks and throw again.
It depends on how precisely you read the angle. Stand still, aim exactly at the floating eye and use the full decimal yaw from F3. Small angle errors grow with distance, so if the estimate is far away, re-throw once you are closer to home in on the staircase.
In Java Edition, 128 strongholds generate in 8 rings around the origin. The first ring holds 3 strongholds, 1,280-2,816 blocks from 0,0 - so if your estimate lands way outside that band, double-check the readings (or you may be triangulating a farther ring).