Lawrence... I've seen many formulae for this but none of them are ever consistently anything but wrong. As has been said already, the published range is based on lots of variables. One of the biggies here is signal field strength. In essence, the greater the field strength, the better the range.
The trouble with this is that field strength has many variables, so you can end up with vastly different strengths in different approach sectors. The published figures can take this into account by either averaging the strength throughout the whole azimuth, or by defining sectors with different ranges.
Now, the actual range at which you'll pick up an identifiable signal may vary considerably from the published range. But, if the navaid is sited in an area of poor conductivity - and especially if it has a weak earth mat - the actual range may be only a mile or two more than the published range.
But the point to remember is that your enroute LSALT, MEA or whatever you want to call it, could be based on that navaid. In that event, it's unwise to rely on the signal beyond the published range. And, as you may not know whether or not that is the case on any given track, it's generally not a good idea to trust the signal while you are outside its published range.
This is most especially true for Locators, because of their much lower power output which, in turn, results in a generally much lower signal field strength. I know this hasn't actually answered your question, but it IS the way things are with this type of navaid.