The G25 Load sensor mod will calibrate much like the standard pedal. The difference will be when you are asked to move the pedal throughout it's range.
At this point you will press the pedal as hard as you think you would like to be braking while racing.
NOTE: If you notice the axis is reversed in your sim, you can reverse the axis in the Logitech Profiler.
If using the Bodnar controller, you can use the very nice calibrating software Leo has made that will allow you to get a full scale reading.
You can get it here http://www.leobodnar.com/products/DFPG25conn/Calibration-DFP-G25.exe
In iRacing, the brake “measure” feature will also allow you to fine tune your braking effort to your liking.
Now for some personal opinions on calibration and racing stuff. Blah blah blah
Compared to a traditional load cell, this load sensor modification is different in that you may prefer a non linear setting. Personal preference will determine how much of a non-linear setting you like.
I feel it's most realistic when you only lock them up if you try really hard to out brake yourself or you are in a panic situation. (like someone spinning in front of you).
This may be hard on your feet and wearing shoes is a good idea. Yes, there are some folks that will prefer a setting that allows lockup with minimal pressure. This may come from not being able to forget the ways of a potentiometer but I do hope that you'll try a more realistic setting at some point.
So, I forget where I read it but would like to slaughter a quote from a famous race car driver. He said what's most important about braking isn't how fast or hard you get on the brakes but how you release the brake. I agree and feel this can be more easily accomplished when you are modulating a large amount of pressure. Essentially your increasing the bandwidth of your foot, ankle and leg to the point where you can release the brake with precision and confidently trail brake.
I encourage you try the different calibration settings. If you have any questions, please don't hesitate to contact me.
Have fun and Happy Racing!