Yes you would need to modify the homing files to position the probe where you want it before the G30 happens.
Not sure why the initial file using variables wasn't working correctly to get to the center. It's possible your M557 grid isn't setup correctly for your bed size.
For the lead screw positions you can estimate by moving the print head as close to the lead screw position as you can get it and then using a ruler to measure the distance from there. It doesn't need to be sub millimetre accurate to work, just close enough.
Then in your bed.g file you would manually move the probe to the nearest position it can reach close to the lead screw and probe there. The guide describes everything needed in more detail.