Having a standard based upon the signal is much more sensible. Something you can measure reliably and is common to all systems.
As usual, different manufacturers have different ideas, and I too think 100% is, or should be, the total maximum movement of the servo.
From a Tx point of view this should be a standard signal state.
What angle that represents at the servo could be down to the servo manufacturers. But certainly the centre point should be clearly defined.
What's that JETIbox all about Aidan?
Sounds interesting.
Ron