Using the VAR knob in the center of the time/div
knob I can adjust the display to show exactly 10 divisions.

Once you move the VAR knob off the cal position you have
lost the ability to measure time on the X axis or peak voltage
on the Y axis.

So in your case you really don't know how long each division is.
Put the VAR know back on its detent and select the best scale.
Then you can read the period off the X axis. If you vertical
knobs are in the cal position you can read the peak voltage
off the Y axis.

You really should find a book that shows you how to do this.