well have not done it yet but the X and Z being added together is a good defalt. I have one on a little difrent set up.. I need a rope to move buy so much for the Z rotait on one joint.. and thin I need it to be moved some more when the next joint rotaits Z.. so its helpfull like that..

For your set up, first thing I think of is to creat a new atribute, and just make a exprestion that takes the X and Z and avriges.. the result going in to new atribute.. thin put your SDK on that.

hope this is a good salution for you.. you can do a lot of new, neet things with a extra atribute and some exprestions.