As was stated before measure the time over a given section. On a previous layout I used a 6 foot section (two lengths of flex track is 1/10 of a scale mile) and multiplied the time by 10. If the resulting time is 60, 60 scale mph.
Here is the chart I used for reference.
240 sec 15 smph
180 sec 20 smph
120 sec 30 smph
90 sec 45 smph
60 sec 60 smph
45 sec 90 smph
30 sec 120 smph
20 sec 180 smph
15 sec 240 smph
Here is the chart I used for reference.
240 sec 15 smph
180 sec 20 smph
120 sec 30 smph
90 sec 45 smph
60 sec 60 smph
45 sec 90 smph
30 sec 120 smph
20 sec 180 smph
15 sec 240 smph