As you said, 50% sensitivity gives you linear steering regardless of how many degrees of steering wheel travel you have or the steering lock you using the setup. Are you trying to match your wheel movement with the one on the monitor? If so why, it's easier to remove it from view and only see your own. Make sure you have speed sensitive steering at 0 degrees.
Steering sensitivity at 50% means linear, move the G27 half of its rotation and the cars tires will move half of their travel, move the G27 all the way and tires will be at max steering lock.
Steering sensitivity at 25% means that if the G25 is rotated half way, the tires will be at about 25% of lock so every additional degree of G27 movement will be more than normal tire movement for each degree of travel as full rotation is still full steering lock for the tires.
Steering sensitivity set to 100% will get you about 75% steering lock with the G27 turned half of its rotation. The next half of the rotation gives less steering lock per degree of G27 rotation and max steering lock is still at full G27 rotation..
All that is easy to see if you on the Advanced page of Options/Controls and look at the bar for steering on the bottom of the page. BTW the same is true for the Brake and clutch sensitivities.
Once you have your G27 rotation set drive the car and set the steering lock in game so that you feel comfortable or in control of the car. While out on track don't forget to drive right over the Steering Ratio Calculator and smash it to bits!
