FYI, there are plenty of sources on the geometry of solar eclipses including the August 2017 event, e.g.
https://www.discovermagazine.com/the-sciences/solar-eclipse-geometry
"Two Types of Shadows
If the sun were a point source, like one of the nighttime stars, the moon would cast only one kind of shadow. Instead, the sun stretches 0.5° across, so even during total solar eclipses, some of its light passes either above or below the moon, creating a less-dense shadow called the penumbra. Only where the moon blocks all the light from the sun — in its dark inner shadow called the umbra — can people on Earth see a total solar eclipse. Anywhere in the penumbra, the eclipse will be partial, but the percentage of the sun covered will increase as you get near the umbra. Unfortunately, the umbra is small, no more than a hundred miles in diameter. On the other hand, the penumbra measures more than 4,000 miles across."
This is an example how astronomy can calculate based upon the size of the umbra and penumbra, where folks will see a total solar eclipse event or partial. Annular solar eclipses take place too but the Moon is farther away from Earth then so does not cover the nearly 0.5 degree angular size for the Sun.
From another source I use that calculated the geometry for the August 2017 total solar eclipse we have:
"Figure 2: The flat-earth ‘model’ cannot successfully explain solar eclipses. First, the sun and moon appear to be the same size to an observer on the earth at the place where the dotted lines come together. The moon must be somewhere between those two lines. But, the size of the shadow will be a mere 114 km at its maximum extent during the Aug 2017 eclipse. In order to generate a shadow that size, and to appear the same size as the sun, the moon has to be very close to the earth and very small: 12.5 km away and 0.13 km in diameter. This simply cannot be true, so the flat-earth ‘model’ must be wrong."
"According to most flat-earth adherents today, the sun is only 5,000 km (3,000 miles) away. To explain both the apparent size of the moon and the observable size of its shadow, there is only one place it could be in the flat-earth model: the “dual-constraint lock point” in Figure 2. It turns out that the moon must be only 12.5 km (7.5 miles) above the earth! And its diameter must be only 0.13 km (about 427 feet)!"