Mixing Joystick Values for mecanum wheels

[Leav]

We are preparing for the possibility of installing mecanum wheels on our robot this year and as a proccess are trying to figure out how to mix three joystick values into a working motor signal...

X is what controls your left/right movement.
y is what controls your up/down movement.
and Z is supposed to control the robot's spin around the Z axis.

we have nailed the combo for X and Y but i'm still lacking the Z integration...

m1/-------------\m2
    |-------------|
    |-------------|
    |-------------|
    |-------------|
    |-------------|
    |-------------|
m3\-------------/m4

m1=y+x-127
m4=y+x-127

m2=y-x+127
m3=y-x+127

any ideas on how to integrate the Z axis so that when stationary the robot can be controlled to turn around itself but can still be turned while moving in a straight line (i realize that the form would be a circle of sorts)

later on i hope to tackle moving on a straight line in relation to the field while all the while turning, so that the effect is somewhat like an ice puck, turning around itself while still moving in a straight line)

but first thing first.... any idea how to integrate the Z axis control?

Thanks,
-Leav