How big is my vanguard/rearguard?

…if I have exactly five divisions in my army? I can’t make them satisfy the rule that each one contain between a quarter and a third of my total number of divisions in that situation, since five divisions don’t evenly into 3rds or 4ths. Should I go for 2 - 1 - 2, or 1 - 3 - 1, or 2 - 2 - 1?

…if I have an army with only one or two divisions? This might happen if a huge army column were broken down to march more quickly. Are they simultaneously in all multiple positions at once?

Round the main body requirement up, round the vanguard and rear guard down.

So you need a minimum 2 in the main body. So you can do 1-2-2 or 2-2-1.