Wayne,
You can set up an even spaced snap grid by going under the 2D Settings menu at the near the top of the screen. * and/or* If you place your cursor on the numbered guide above the work area and to the left, you can drag down a guideline to align your vectors. You can make as many as you want and conversely can drag them back to the top or left to get rid of one.

Place horizontal and vertical guidelines where you want your holes to be placed. Then create your 5/16 circles and drag them over to where the X and Y guidline intersections are. The cursor crosshairs will chance to a 'circled +' telling you that the center of the circle will be placed at the intersection.

If you get to the point where there are too many guidelines on the screen and/or they obstuct the view of the vectors, click the # symbol under the 3D view button to hide the guidelines. Click it again to turn it back on.

Hope that helps.

-Brady