Here is an example of how the grid plug-in works:
This screenshot depicts a four-square grid where each line has a width of 1 pixel. There is no offset around the border. The intersections are 0 so that lines smoothly intersect each other.
The spacing is the part that you seem to be having trouble with. First, you should set it in percent by clicking the drop-down menu next to it and selecting the percent symbol. This allows you to give each line a proportionate spacing that you don't have to calculate. When regarding percentage,
this is considered to be 100% horizontal, 100% vertical. This means that there will be a gap between each line equal to 100% of the image's width and height. If you take 100 here and divide it by 8 (as in your case), then you will get the number of lines you wish to have expressed as a percentage (in this case, 12.5%). If you insert 12.5, 12.5 then each line will have a width and height of 12.5% of the total image, so that you end up with an 8x8 grid. This method works for other grid sizes. You can click the chain icon below the spacing category (as seen in the first screenshot) to set the widths and heights between each line independently.
For example, if you wanted a 5x3 grid, you would use:
100 / 5 = 20 for horizontal lines, and
100 / 3 = 33.33 for vertical lines.
The screenshot below illustrates this.