Q1: float diameter_mod = 1.24 is modifier used for ROSbot odometry. When turning, real rotation angle is lower 1.24 times than determined by encoders. It is caused by wheel slippage.
Q2: It is encoder resolution multiplied by gear ratio: 48*30613/900 = 1632.693. You can find details in ROSbot manual.
Q3: This is wheel max speed in encoder tics per second. It roughly equals to ROSbot max speed.
Did you determine the real rotation angle through testing, or is there a way to estimate it through the kinematics of skidding. If so, how would I be able to estimate the real rotation angle?
We determined this value through tests. It may vary, depending on floor surface.
If you want to get most accurate odometry results, you will have to experiment in your environment.