I know it sucks and it is not the answer you are looking for but...
Trial and error is the only method I have ever had work.
You can use a string to get an idea how long it should be but remember the string will be sitting in the grooves of the pulleys. This seems to screw up the measurement.
Mine is 73.5 inches.It was not the GoodYear belt,though.It was the Duralast from AutoZone.That's no a/c,no smog pump (or smog pump delete,but just completely bypassing the smog pump),a/c delete kit from Ford Motorsports.