Getting the center-to-center distances and positions right-on can be tricky. 1) You can use the grid as a snap too to get them right, which may involve using more than just two grids; 2) You can use the "information" tool to adjust positions; and 3) You can use the command line to set the positions.
If you are a very new user and not familiar with the command line, options #1 and #2 are probably easiest. Eventually, if you do much of this, you will find the command line is easier.
John