I think that you'll find that your DA-50 is getting most of its power in the first 1/2 to 2/3 of the throttle server (butterfly valve) movement, and there is little to no RPM increase beyond that point. I use a 13 point curve in my 9z with a noticeable flat spot that brings center stick down to about 25-30% of the servo travel. Then the curve moves up steeply past the 70% point. This curve has proven to be very linear on the RPM increases on my DA-50 using a KS-1060 tuned pipe and a Mezjlik 23X8 prop.

What Tx are you using? Do you have throttle curve capability?