Corey, line of sight (LOS) calculation is nothing but the math of physics. It is a geometrical formula that determines the distance at which the tips of the antennas have a clear, unobstructed path between them, assuming no other man-made or natural obstructions.
The actual LOS could be much less, but never more! The earth is round after all. It would be so much simpler had the earth been flat... <snicker>
Rather than bore folks with the complex math, here is a screen clip of an LOS calculator. I was a bit low because I was relying on my faulty memory. The actual LOS is more like 17 miles. Note that this assumes that the receiving HT or mobile has an average height of 6'.
Screen clip courtesy of http://www.hamuniver...calculator.html
Now LOS is not an absolute predictor of actual propagation distance, as your observations illustrate. LOS assumes nothing is in the way of the radio signal between the antenna and the horizon at a chosen height above ground. It does not take into consideration any attenuation caused by weather, band conditions, antenna gain, path loss, or other factors such as dB loss in coaxial cable.
Likewise it doesn't consider any favorable circumstances such as signal refraction, reflection, or tropospheric scattering. Nonetheless, the physics and geometry do provide a solid basis for best case performance.
By the way, the actual LOS calculated for your 150' AGL is 20 miles. So your "guesstimate" of 25 miles is reasonable. Keep in mind that the higher the repeater's antenna, the more it is likely to "shoot over obstructions".
One neat site for generating a predictive propagation map is found here: http://www.ve2dbe.com/rmonline.html
As indicated on the Radio Mobile Online website, it uses digital terrain information and a mathematical model to simulate radio transmissions between two fixed sites or between a fixed site and a mobile. The digital terrain information comprises three databases: ground elevation, land cover, and population density, which combined total 200 GB of information.
Since the site is in French, a good English set of instructions may be found here: http://ham.stackexch...-for-a-repeater
Here is a screen clip from there for my repeater. I've been using this map as I drive around the area making tests. I've been very conservative in my estimates, figuring it's better to be surprised than disappointed...
One thing is immediately clear from this. I really should have a more directional antenna with the azimuth aimed southwest. I'm "wasting" a lot of my system towards the northeast and the bloody lake!