Tread to riser ratios for outdoor steps.

Typical tread and riser dimensions for outdoor steps

Two considerations: Basic comfort and disability recommendations.

Based solely on comfort and ease of general use, optimum step size appears to be 7.5" risers with 9.5 to 10" treads.

ADA site: http://access-board.gov/ada-aba/comparison/chapter5.htm

ADA requirements are for risers not to exceed 7" and for treats to be at least 11". This makes sense. Here's the language from the ADA building requirements.
504.2 Treads and Risers. All steps on a flight of stairs shall have uniform riser heights and uniform tread depths. Risers shall be 4 inches (100 mm) high minimum and 7 inches (180 mm) high maximum. Treads shall be 11 inches (280 mm) deep minimum.

The 7" riser is a good mark. It could be a little thinner. And the 13" tread recommendation for flagstones is a valid mark.

The 7"-11" ratio gives the right "rule of 77" result. But this may mean more steps. 7" by 13" may do better. See the note that says on twisting stairs the tread depth is minimum 11"@ one foot out from the tight end.

Also see the note that says the riser high can vary by no more than 3/8".

A 3 foot wide step with base is roughly a 1.7 cu ft volume of concrete.

http://teachers.henrico.k12.va.us/math/HCPSAlgebra1/Documents/6-1/SlopeStairsLab.pdf
Notes on code for stairs. Interesting ratio formulas.
Shows twi formulas for stairs.
r+t ~ 17.5
r*t ~ 77
That says for a nominal stairs, t = 17- r
-r^2 + 17r - 75 = 0

Riser = 8.5" max = 6.5" min. Nominal riser = 7.5" (Except ADA says 7" max)
At 7.5", optimum tread depth is about 10 inches.
At 6.5" optimum tread depth is 11.5"

This also fits another formula from the source below:
2R+t = 24.
6.5" x 2 + 11.5 = 24.5

http://books.google.com/books?id=SplcE_LjuFMC&pg=PA57&lpg=PA57&dq=steps+tread+riser+ratios+OR+slope+-patent&source=bl&ots=e2-GmxW2yA&sig=MGBhGQOh19ExDJSe1nmQvN4Nm8w&hl=en&ei=dzXnSsHTDcbU8AaZ1JyIBw&sa=X&oi=book_result&ct=result&resnum=10&ved=0CCcQ6AEwCQ#v=onepage&q=steps%20tread%20riser%20ratios%20OR%20slope%20-patent&f=false