Calculate the Top of a Cloud
The formula you found comes from the barometric formula. It assumes a hydrostatic atmosphere (no vertical accelerations), a constant temperature with height (or a mean temperature) and uses the ideal gas law:

$$h= -\frac{RT}{g} \cdot \log \frac{p}{p_{sfc}}
$$
where h is height above ground, R is the ideal gas constant, T is the mean temperature, g is the gravitational constant, p is the pressure of the cloud top in your case and $p_{sfc}$ is surface pressure (not reduced)

The first factor comes down to a value of $-8$km if $T \approx 273$ K. This value is also known as scale height. $p_{sfc}$ is approx. 1013 hPa if you're at sea level.

So LOGS would most probably mean the natural logarithm.

I don't think there is a simple equation for the cloud top height from ground observations because e.g. how high a cumulus grows depends on the temperature of the surrounding air.

Cloud base height and cloud top height are two totally different things. The cloud base height formula does not depend on the on the upper air temperature. 

Whether a thermal (which evolves into the cloud later on) reaches its cloud base or not, is not sure. As long as it's warmer than its surroundings, it rises. The same for the cloud: It rises up as long as it is warmer than its surroundings. You may know what's the temperature of the cloud at each height, but how cold the air at, say 15,000 ft is, varies a lot.