How to calculate stair stringer when total RISE is small?
Yes, there is a ratio of height of riser to width of tread. Exterior stairs are not generally as steep as interior stairs. The ICC Code requires a minimum of 4” high riser and allows residential stairs to have a maximum of 8” high risers. All risers in a run must be within 3/8” difference of largest to smallest. (See R311.7.5.1.) 

A commonly used formula for calculating the depth of tread to height of riser is established in “Graphic Standards”, commonly used by architects and engineers. On page 7 they give this formula for interior stairs:

Riser plus tread = 17.5”.

Thus 7 1/2” riser + 10” tread = 17.5”

Or

Riser times tread = 75

Thus 7.5” riser x 10” tread = 75

You have two steps 6” high...so I’d use 12 1/2” tread.