I am by no means an expert in casting, I am only starting to dip into switch rods
With that said, check out youtube. From the looks of most videos I've watched, and recalling my instruction, it seems like you want to be at the top of the grip. By all accounts that I've read, these are some of the masters when it comes to two handed casting.
[ame=http://www.youtube.com/watch?v=pNoMcWGoY8g]Advanced Speycasting with Eoin Fairgrieve - YouTube[/ame]
[ame=http://www.youtube.com/watch?v=H5V1vxR2cr0]Master the Scandinavian Speycast with Henrik Mortensen - YouTube[/ame]
[ame=http://www.youtube.com/watch?v=yKweQaWEQg8]How to Cast a Floating Tip on a Skagit Line---From Skagit Master Volume 1 Featuring Ed Ward - YouTube[/ame]
[ame=http://www.youtube.com/watch?v=Ua3JT3gmsjI]Simon Gawesworth at SOR, Spey casting. - YouTube[/ame]