Use a Tachometer, take a reading at full throttle. Set your throttle stick at 1/2 position on the TX, then use your multipoint throttle curve to adjust the mid stick position downward, until the RPM is approximately half of the rpm at full throttle. Leave the rest linear to begin with, and then go back to fine tune it to your feel, bits at a time. This will get your throttle feeling more linear from idle to full throttle, and get you away from the characteristics you describe.