Don't see any problem with the rounding up interpretation. By the time a mech is wrecked that far, it's gonna want to leave anyways. Forced withdrawal is supposed to represent some form of intelligence, self-preservation instinct, financial responsibility, fear of dispossession, etc. If you aren't playing zombie hoard mechwarriors, do you really want to be running your Jenner around with only two structure? If rounding up is a problem, then why use the FW rules in the first place?
Morale checks to override forced withdrawal is a whole different matter, which I may not be opposed to. But that topic belongs in a different subforum.