Hi - what you want to do is use a Program Mix (P.Mix)
Change from INHIBIT to ON
Leave as GROUP
Set Throttle as MASTER
Set Rudder as SLAVE
Where you see the graph you can select a curve type (MODE)
Play with it to find a curve that gets you what you want.
I'd probably use a POINT curve to keep the rudder trim at zero until you hit the amount of throttle you want to start the mixing at.Then adjust the points as you wish for the amount of rudder you want. The curve will be mostly flat at the center of the graph, and then have a slight bit of offset as you get to 70% throttle.
I use this method for mixing down elevator with low throttle for aerobatic planes to keep the nose pointe straight down on downlines. Most planes tend to pull to the canopy on longer downlines. My mix stops that.
You can also do this another way and set the curve to a flat line with the amount of mix you want and use the throttle stick position to turn the mix on/off.
Are you mixing in rudder above 70% throttle to trim out an issue at higher speeds, or for take off?
For more info see page 76 of the manual:
Also look at pages 120, 123, 124