The horizon makes a 90 degree angle with the Earth's centre.
This is because the astronaut's line of sight makes a tangent with the planet
(if you balance a perfectly round ball on a table, it's centre is directly above the point of contact or tangency).
The triangle has the same sides and angles,
so the trigonometry works exactly as skeeter originally outlined.
Sorry Skeeter, my mistake 380 miles is correct
Thank you so much, I get it now and I completely understand, Thank you Skeeter and Archie