Thanks for the feedback! I think you have a good point about slow units. The mission should probably go on until at least round 5 to give them a better chance of completing the mission. Perhaps they could get around it by using APCs. I don't think that would break the game, honestly. The attacker has plenty of ways to block the movement or stop the APC in some other way.
With the percentages, I did mean the number of units. It's also not specified in the official rulebook, so I tried to keep it consistent with that. However, it is clearly causing some confusion. Perhaps it's better to specify it in the mission anyway.