Well, I screwed it up…or actually down. I was playing with my cork and got it all out of whack. Can anyone tell me the proper distance from the cork to the center of the embouchure hole for the Forbes (and I’m assuming Prattenesque model) flute?
^I measure roughly 23mm… but I’m only half-way through my first cup of coffee.
… mmmmmmmmmmmmmmmmmmmm - coffee! Good mornin’. 
Back when I got my own Forbes flute, there was an option to have either a standard, fixed-in-place end cap cork, or, an optional, adjustable end cap cork. So, because I like having a choice, I got both, one of each. However, what I discovered was that the standard, fixed-in-place end cap cork just happens to work delightfully well, and I have never actually used the optional, adjustable end cap cork.
So, I’d suggest that you contact Rob Forbes, and place an order for the standard, fixed-in-place end cap cork.
Really, it works just fine, and you’ll never have any cork displacement troubles again.
Edit: SEE POST BELOW FOR MEASUREMENT
Cork, since you know what the maker recommends, why not tell the original poster what the distance is?
Best,
Hugh
OK
After some measurement, it appears that the distance from the face of the Forbes flute “cork” to the center of the embouchure hole is approximately 20.5 mm.