Welcome to gas! It's not just the 20, they're all this way. Even string trimmers and leaf blowers! Fortunately we've learned how to deal with this.
The diagram he's talking about is here: Newbie to Gas General Information
Drop down to the bottom of post #34. It illustrates how you can achieve a linear throttle using mechanical differential. Many suggest you start using the 50/50 setup you would use on a control surface. Thinking you're here because you've found out how well that works. It doesn't.
Of note is that the position of the servo in that first/top section demonstrates the point well, but if set up like that, it's difficult to adjust your idle speed or set your throttle up so the engine can be shut down by closing the throttle. Instead, start with the servo positioned as it is in the second section of the drawing. Then you should be able to adjust you idle speed, as well as maintain throttle linearity.
You have a throttle curve available? Not familiar with your radio, but that's certainly an option here as well. Many guys start out with the mechanical differential, then trim as desired using their curve. You would want to set it up so the carb is only about 50-60% open at 3/4 stick. Hard to help much more without knowing more specifics about your radio. Just remember there are positive as well as negative values that can be entered. If you're struggling, you may not be using the right direction? -Al