According to the test motor board program at https://github.com/UbiquityRobotics/ubiquity_motor/blob/noetic-devel/scripts/test_motor_board.py (line 101), the lower bound of the speed that can be commanded to the Magni is -254 (by using this program). However, since 254 is 00000000 11111110, -254 is 11111111 00000010 in Two’s complement. The LSB does not correspond to the LSB computed by this function, which is 00000001.
I think the LSB should be evaluated using 256+speed for negative speeds. This means the lower bound should be -256 instead, since the Two’s complement of -256 is 11111111 00000000.