In addition to what Echo said, you'll see the guide coordinates as you
Ctrl-drag them to create new guides if your Ruler is visible (View | Ruler).

If 8 guides are not sufficient, you can create guide lines with PowerPoint's
line tool -- and then add a dash style to those lines. Create variations of
thick and thin lines to create major and minor guides. These lines can then
be grouped -- and cut, copied, and pasted over any slide you want to use the
same "guide-map" for.

You can also copy the guide-map between presentations -- delete this
guide-map when done.