Guides Offset paper top or sheet between the row and columns

I did discover the Guide feature and it works fine. However, if I use a distance, for example, Hor 100px and Vert. 100px, then the pointer shows me '72,72'. So I assume the 100px is measured from the outside of the sheet and not from the top/right side of the left corner or from the bottom/right side of the columns/headers/rows.

Is this on purpose? If I need some guide repetition, then I have to know the size of the headers and column thicknesses.
Is there a secret setting to change this behavior, or what is the common practice to handle this?