This section can be found at the very bottom of the page - after Additional Charges.
Currently there are two options available to manage weight more flexibly:
Weight unit multiplier - can be used to change weight units.
For example, setting this to 0.5 will let you set cost per half kg/lbs.
Weight rounding - this will round weight up to full weight unit, as per set in the previous option.
For example, if cart weight is 0.35 kg/lbs and weight unit multiplier is set to 0.5, cart weight will be rounded to 0.5 kg/lbs.