In order to get the full range out of your servo's, they must be installed in such a way that the entire servo range will map on the entire throw of the flaperons. This means that you NEED to have some offset on the servos to get to the neutral (cruise) position of the flaperons.
This is shown in the next image:
This implies that the maximum throw around the Offset for the ailerons is equal to (100-Offset).
This implies that the maximum throw below the Offset for the flaps is equal to (100+Offset).
The JustFly models are all configured to automatically correct the maximum throws based on the above formulas. This means that you will never have to apply differential or adjust for on the rates to compensate for a change in the Offset.
If you have a 'normal' servo install, with the control horns on the bottom side of the aileron, the situation is exactly the same. It may only be that you have to inverse the servo direction. That means that -100% will become 100% and vice versa.
We have used the term 'travel' to refer to the % of maximum throw that you want on a certain control surface, such as flaps or ailerons. They are always relative to the max throw. With flaperons, the max travel for flaps and for ailerons do not influence each other, and can be set fully independantly.