I'd like to add something about your dozens/columns matrix.

If we would use 3 x 3 matrix in order to cross reference rows and columns it would look like this:

1,2,3 horizontal rows as reference to dozens.

A,B,C vertical columns as reference for columns.

Let's say that the last spin was number 24, it belongs on 2nd dozen and C column, write on the top left box 2 C.

Since there are more ways to have 2 different dozens and columns on 3 spins we should bet for the other 2 dozens and columns simultaneously.

So the bet would be dozens 1,3 and columns A,B, total 4 units.

For the example's sake the next result is 21, which belongs to the same dozen and column, we lose 4 units and you write on top middle box of the matrix 2 C again.

There 9 boxes for equivalent total of results, out of 9 spins we are betting max 8 times, from those 8 we are expecting 1 simultaneous win for dozen and column, the payout is x2 net.

Divide -4 draw-down by 2 net payout = 2 but since we want to win rather than break even we would bet 3 units by 2 dozens and 2 columns = 12 units bet.