If I set the grid size to 8 pixels (or any number less than 10), the gridlines will draw at every 10 pixels regardless.
I recommend that either...
...the grid size value entered by the user gets corrected to 10 after inputting a lower number to inform the user that the grid can't go any lower than 10 OR
...fix it so that the gridlines are drawn in the size specified by the user.
I hope the latter gets implemented, because as a lo-res game dev, 8 pixel boundaries are pretty important.
Is something not working like it should? Let us know.