What Periphaeria says is right. When you add rows or columns to a design, you say where to insert them. You can do it at the front (left), the end (right) or anywhere in between. You give the column or row number where the new rows/columns are to be inserted.
You do not have any control over the count of the heavier grid lines. You cannot say to start at column 3 and then every 10. You can have it every 10 but it starts at the front.
thanks for the information, I suppose I meant the dark grid lines, as I am copying a pattern and I found it confusing because their heavy grid line started in column 3. But if I can't do it then I will have to make do. But thanks for the info.