RE: MAC calculator?
I take it that you intend to pivot the tail around the Aerodynamic Centre so that there is no pitching moment, no hinge moment? And that is why you want to find the MAC?
Remember that although we generally assume that the AC is at 25% of the MAC, it can vary by 2 or 3 percent depending on all sorts of things.
The safest thing is to pivot the tail just ahead of 25% (say 22%) so that when unpowered (servo passive failure) it will stream with the airflow. That allows the aircraft to be safely controlled by just the tail onthe other side, with luck and adequate stability. It means that the servo is overcoming a hinge moment, but it is still small. It has a high degree of aerodynamic balance.
The advantage is that the tail cannot go aerodynamically unstable, as would happen if the pivot were behind the Aerodynamic Centre.
The location of the pivot is not super-critical, but would err on the slightly-forward side.
What is more important is to mass balance the foreplane, at least partially.
A bit of lead in the furthest forward point of the foreplane, or tailplane, will help prevent flutter. And it is important to have all the control linkages strong and rigidly connected.
See my photo of my Eurosport foreplanes, one with lead in the LE, the other yet to be done.