Slope/Intercept for Groups

I have a spreadsheet of which I have attached a small version.

I am trying to find a way to calculate the slope and width to get a linear forecast of the weekly sales for each Group at each counter width.

I have done it manually in the example, but would like to be able to use a formula or VBA to count the number of entires for each group in Column A and then calculate the slope, intercept and forecast in columns D, E, F without having to manually change the row numbers in columns D and E for each change in the Group

The number of entries for each Group at each counter width will vary.

Thanks