Assumptions:
(1) The tubes to be centrifuge are of the same type and filled exactly the same.
(2) If the rotor has less than 6 positions then the number is even (2 or 4).
(3) If the rotor has >= 6 positions then the number is divisible by 6 (6, 12, 18, 24, etc)
The only time that a balance tube is necessary is:
(1) there is only 1 tube
(2) there is n-1 tubes for a rotor with n positions.
If the number of tubes is even then put tubes in as pairs, one opposite the other.
If the number of tubes is odd, then:
(1) put the first 3 in to form points of an equilateral triangle.
(2) put subsequent ones in pairs, one opposite the other.
