As Echo says. Another way to measure in PPT is to view guides. Drag a guide
to the start of the point to measure from and hold down shift while dragging
the pop up will indicate the measurement.
Drag a guide to the left side of your slide (or Ctrl+Drag to create a new guide
as you drag).
Now Shift+Drag the guide and watch the position readout; it now displays the
distance you've moved the guide from its starting point rather than distance
from center.