Oops too much wine last night! Both you and Bet are good posters.
lol. Don't try to use that excuse on your wife/gf! Anyways, the respect is mutual.
Fair enough, if your experience is different, in terms of needing to go faster to hit the fence. It may well have to do with conditions, eg. court surface, humidity, altitude, just how "flat" your flat ball is, the angle etc...I certainly notice a difference between courts. Nonetheless, when serving with radar, I find that I can hit the fence (not impressively by any means!) with a very, very weak serve. You can do it with a solid "pat", so to speak. On the other hand, sometimes I find the depth/height of my bounce on the fastest serves does not always translate to mph. I can hit a flat 100mph and have it bounce up 4 feet on the fence, fairly powerfully or hit a 120mph serve and have it hit 3 feet up (perhaps a bit more powerfully). I usually think this is due to a bit of spin being picked up.
In any case, I use type of impact on the fence to tell me some things(basically trying to get the type of bounce I want for that serve and a bit of feedback on the spin) in practice, but I'm not sure it has much value as a judgment of speed measure....maybe +/- 10-20mph....but really, if you have a real (not the imaginary, "I'm sure it is" big serves here ;-) 100mph serve, you can already make a closer judgment than that!
For the non-big server who wants feedback on his speed...well, a clean 80mph can hit the fence easily....I also think 60mph can....but in any case, all I can really say based on that alone, is if you AREN'T hitting the fence, we have a problem!