[ADTech]

You're playing with values of antenna elevation that are a fraction of the resolution of the vertical elevation data used for the calculations. TVFool uses either the 1 or 3 arc-second resolution STRM elevation databases which "averages" either a 30x30 meter or a 100x100 meter block of terrain into a single elevation data value which is then used for the subsequent LRI calculation. Unless you're dealing with relatively flat ground or slowly changing terrain, the resolution of the underlying data is too coarse to support small changes in the terminal elevation value. In addition, the TVFool calculations do not include the vertical elevation pattern of the transmitting antenna, nor electrical or mechanical beam tilt for those antennas nor does it account for any ground bounce effect.

GIGO. In other words, such an exercise is for entertainment purposes only.

Use the software tool as a baseline estimate, then get metal up in the air and search for a spot where the signals exist.