How to calculate line of sight from an antenna?

We know that higher bands like VHF/UHF and above have line of sight propagation, meaning that it will not be heard beyond the horizon where the Earth’s curvature sends the signal off to space! But there is some difference between actual radio horizon and visual horizon. Usually radio horizon is a bit beyond the visual horizon. The question came up recently when there was a balloon CubeSat launch from South India. I was located about 275 km from the launch site and wanted to know whether I will be able to receive the signals from the CubeSat. Maximum predicted height for the balloon was mentioned as 32 km. Beyond that the balloon would expand and rupture as the air in the atmosphere there is quite thin compared to the helium gas in the balloon. The CubeSat would then be brought down by a parachute which will be deployed at that moment.

On the launch date I had used an online calculator to find out the actual and radio horizon. Now I have come across the formula for calculation of the line of sight from the tip of the antenna or balloon height in the case of balloon CubeSat. Multiply diameter of the Earth by the height of the antenna and take the square root of the product and you will get the line of sight or horizon. The value which I got using this formula matched with that I got using the online calculator! It was 638 km. Still I could not receive the signals, either because my setup was not good enough or the CubeSat might not have risen to the predicted altitude.