Well, I am going to disagree with those saying not to use the expo.
Since your throttle opening is linear to throttle stick movement, I see no reason not to experiment with the expo to see if you can tune the rpm to match the throttle stick position.
While I think the method Minnflyer showed you with the servo arm positioning is a good place to start, it will only get you part way there.
I say copy your setup that has now been test flown into a new model memory and try adding the expo to the throttle on the new memory location, that way your original setup will be intact and your experiment won't cause you any grief if it does not work.
The worst that can happen is it won't work the way you want it to.
If that is the case just blow the model out of that memory location and you have lost nothing.
I am surprised that the 7c does not have an adjustable throttle curve in the ACRO programming, but my quick look at the online manual indicated it was a HELI only program.
It was years before I had a radio that had the adjustable throttle curve, I did what I could with the mechanical expo but there has always been a lot faster throttle response at the low end. When I finally programmed in that linear response I found out I did not like it at all.
Maybe it is because I have never had linear throttle response, but it did not suit my tastes so I went back to what I am used to.