Laying Out Stair Stringers
Unless I'm wrong, the math is pretty simple & straightforward.
First, calculate the total height of the stairs (from the ground to the top of the deck). For instance, 27 inches. Begin by dividing roughly by 7 - (approx: 3.857). Since you can't have .857 steps, round up to 4. So you're going to have 4 steps. Next, calculate how much the rise is for each step. Take 27 (inches) and divide by 4 (steps). This will give you 6.75 - so this means the rise will be 6 and 3/4 inches.
In the end, the stringers should have 4 steps @ 6 3/4". You're average tread ranges from 9" to 11". This is up to you really. Also take into account for the 1" bullnose for each tread (1" over hang per step).
Note one last thing - since you know you'll have 4 steps, you must subtract one step for the top of the deck. In the end, each stringer will have 3 steps. You could have 4 steps in the stringer but the top step will be coplanar to your deck and may look (& feel) slightly odd.
One thing worth to note as well, if you used 2x8's as boxing for the base of the deck then you'd want your tread to match & come up flush to the top of the deck. Since a 2x8's real width is 7 1/4, you may want to consider the rise be 7 1/4" to match. 'sup to you.
This at least should get you going.