Too much calculatin-- not enough doin--
The relative arm positions -if both start at center of a given arc- both change at the same rate
The arc of each will be different but still the rate of change will be the same -so forget the math studies here .Unless you would rather fiddle than fly -

what you are after is maximum use of servo power per angle of surface deflection

In actual practice- matching the throws of both elevator halves is far more important and will be much more easily done if you maximize the throws of the servos
setup at 100 degrees or 120 degrees - I will bet money you find the higher throw (servo) more accurate .
All of this is assuming you have two reasonably well matched servos on a radio which really matches outputs.
some very expensive elaborate TXs rare not the greatest at this. also those shi tty servo reversers and cheap extensions can kill an otherwise good setup