That's because you do not use the offset function "0c".
Here is how you should do it. 0c is specifically designed for flaps.
In the example below i used a takeoff position of 15 degrees and offset the control by -15 degrees.
The deflection then becomes 45 down to keep the max deflection of 60 degrees.



Note that the torque requirement for a deflection of 15 degrees at 75 mph is 61 oz-in. This is good.
As you deflect your flaps towards 60 degrees you want to slow down, as 75 mph would require 257 oz-in of torque from the servo at least.