Adjusting drivers with power on - or even a large static charge from a metal tool - can kill them. Never turn the voltage dial with power on. Never plug/unplug motors with power on. Never plug/unplug drivers with power on. If possible use a plastic tool.

With power off, install the drivers on the X and Y axis of the RUMBA board. Note the voltage dial at the south side. Installing a driver upside-down will kill the driver.

Connect power to the robot, and get a multimeter. Turn the multimeter dial to 20v resistance.

Put the red lead of the multimeter on the dial in the bottom center of the driver.

Put the black lead on the bottom right pin of the driver. The target value is 0.17

Very small changes (5-10 degrees) are all that are needed to make big changes to the target value. a plastic flathead screwdriver works great.

Turn power off, adjust the dial, turn power on, measure the value, repeat until within +/-0.05 of target.