I would round up, but the point is moot if guard class.
Once a unit has passed its test to
charge a 6 can no longer auto stop them in the next test. (baring casulaties to the unit) The second test is for receiving shooting casualites and merely a morale test and not a test to charge as they are in the action of charging.
So with a guard unit with 5 or 6 figures cannot fail to charge home, ie fail their morale test. 6 figures testing as guard roll a 6 minus 1 = 5 pass, 5 figures testing as guard roll a 6 minus 1 = 5 pass. A guard unit would have to have lost 2 figures before it has a chance to fail.
As I understand it, I am still wrapping my head around these rules as well.