Well, basically, what you want to do is adjust your Dual rate to full travel (whether that is 100% or 125% depends on your radio, I'm not sure about the M8). Then set the steering EPA so that you get maximum steering without making the servo try to travel further than it physically can.
This way you can just set the D/R on the fly as opposed to setting the EPA every time you want to adjust the amount of steering you have.
I'm not entirely sure as to how much experience you have, but I would say that you should be using more than just 40% of your vehicles maximum steering throw. If you have to do that to make it work, something is wrong with your driving or your setup. If you have problems with spinning out, and you use a one-way, dial out some brake EPA and make sure you aren't braking as you are turning. If you've already done this, check with other guys that have the MTX3 at your track for a setup. And possibly let them have a go with your car around the track a couple of times to make sure it isn't a driving thing or some glaring problem that you must have missed.
Good luck, and I hope I helped a little.