There is an easier way to set up a matrix. I can't remember if I found it in a post or figured it out by trial and error. I am not at the computer that I sequence with so this is from memory.
Start a new sequence and create or insert the smart string controllers. 3 controllers with 120 nodes in your case.
Right clik on one of the controller tabs. About half way down click on draw images.
On the draw image screen select the matrix drawing tool and draw a matrix on the screen at the location where you want it. The matrix tool will allow you to select the range of channels to assign to the matrix.
Select channel range then sleect multiple or single pixels. then click next.
The next screen should let you set the size of the matrix. (12 x 30, 30 x 12, etc.) It will also give you a choice of 6 paterns to use for the order of the channels in the matrix. Set size of matrix and select pattern then click next.
The channels will be assigned to the points on the matrix. It will save the matrix with the date and time of creation.
I am not an expert, but this method has worked for me and does not take much time. You may have to expermient to make it work correctly.