Hi Jo,
If you are in the early stages of design, why not download the Bootstrap extension to create the site. The idea of having a certain number of columns for different screen sizes is not what you want. Bootstrap says that all screen sizes have the same number of columns, i.e. 12 columns. The columns are wider as the screen size changes up. In some instances you may require the full 12 columns do display the information, in wider screens you may only need 6 columns to display the information allowing for other information to sit in the 6 columns beside it.
If this is not the advice that you were seeking, please come back here and I'll analyse the existing page.