This is an area where the phrase “close enough for government work” comes into play. The fact that one foot per second is approximately 60 miles and hour makes rough calculations very easy. As was pointed out, the longer your measuring zone is, the more acurate your measurement. Just find the longest straightway on your layout and you can get a pretty good idea of what a various scale speeds look like and go from there.
Most model trains are probably run too fast given the conditions we run under. A real train might highball at 80 mph but that is going to be on a long straight section which is pretty much unheard of in the model world, even on large layouts. I actually have one long straight away that is a scale half mile long and that is very unusual. Given the number of 90 degree turns and grades that are the rule on most model railroads, it is rare we should even run at 60 mph even though your track and trains can handle it. The way I look at it, the slower I run my trains, the longer my layout will appear to be. The fastest I run my trains is about 40 mph.