A scan of the literature shows that numbers for the same type of device vary considerably from one report to another. Reported parameter values are not guaranteed maximums but are usually averages of a few parts tested. Like propagation delays, metastability parameters vary with process variations, voltage, and temperature. Small variations in the time constant t, especially, cause enormous variations in calculated MTBF because t is in the exponential term. The material in Ref 4 discusses the problem of parameter variation, giving an example in which a typical MTBF of 317 years shrinks to 12 minutes when you use estimated worst-case values.
Calculations are useful for getting a rough idea of the magnitude of the metastability problem.
Following basic principles helps you to minimize the problem. The most important principle is to allow as long a time as possible for metastable conditions to settle. This method reduces t', thus having the same effect on the exponential term as doubling the clock frequency. Simple guidelines can gain a few nanoseconds, which may translate to many multiples of t. Second, you can reduce the effects of metastability by using a multiple-stage synchronizer, which adds stages of pipeline delay.