I have stumbled onto something that I don't know how to accurately
measure.

I've got a low deck (21" above ground), and I am trying to cut 2x8
stringers to enclose the two required steps. I probably need some
support in the middle as well (3' span).

I want to make sure that the ends of the stringer are flush to the
deck and to the ground. Side view would look something like this:

| -- side of deck
| \
| \ -- stringer
| \
| \
| \
________________\_______ --- ground


Ideally the angle between deck and ground would be 90 degrees. Any
ideas on how I can measure this accurately?