Pete, I get very confused about the thickness factor( I really don't understand it!) But you helped me , when you stated the weight of the dough. I started out making my pizza with 15 oz. of flour( weight was about 24 or 25 oz.). As my stretching technique improved , the pizza got too big so I reduced the flour. I guess it's trial and error. I want a thin to medium crust. I do like the 13 oz recipe but sometimes I don't stretch it out evenly. Practice makes perfect but I want to know when it will get perfect. I've been making a saturday night pizza for the last 20 Years ! It was only until I stumbled upon this site that it all came together.
Kate,
I wouldn't be too concerned at this point about the thickness factor. The thickness factor is just a way of determining how much dough to use to make a particular pizza of a given size. The value of the thickness factor will vary by pizza type because not all pizzas have the identical crust thickness. The technical term for thickness factor is "dough loading" factor. That is the expression that Tom Lehmann uses. I decided to use the expression "thickness factor" because I was afraid that peoples' eyes would glaze over or they would nod off if they saw the expression "dough loading". If you ever get to the point of using the dough calculating tools at
http://www.pizzamaking.com/dough_tools.html, which incorporate a Thickness Factor option, that would be the time to become more familiar with the thickness factor concept. In the meantime, if you are interested in knowing more about the thickness factor, you might read and study the thread at
http://www.pizzamaking.com/forum/index.php/topic,17813.msg172515.html#msg172515.
In case you are interested, this is what your recipe looks like in baker's percent format:
KASL Flour (100%): Water (61.5385%): IDY (0.40865%): Diamond Crystal Kosher Salt (1.84506%): Olive Oil (0.30525%): Sugar (2.16346%): Total (166.26092%):
 368.55 g  13 oz  0.81 lbs 226.8 g  8 oz  0.5 lbs 1.51 g  0.05 oz  0 lbs  0.5 tsp  0.17 tbsp 6.8 g  0.24 oz  0.01 lbs  2 tsp  0.67 tbsp 1.12 g  0.04 oz  0 lbs  0.25 tsp  0.08 tbsp 7.97 g  0.28 oz  0.02 lbs  2 tsp  0.67 tbsp 612.75 g  21.61 oz  1.35 lbs  TF = N/A

Note: Dough is for a single 18" pizza; the corresponding thickness factor = 21.61/(3.14159 x 9 x 9) = 0.084922; no bowl residue compensation
If you decide that you would like to change the percents of any of the above ingredients, for example, to lower the salt to 1.75% or 2%, you can use the expanded dough calculating tool at
http://www.pizzamaking.com/expanded_calculator.html to do so. You just change the percents. If you decide that you would like to try a higher or lower value for the thickness factor noted above, then you can use the Thickness Factor option of the expanded dough calculating tool. You can also use the bowl residue compensation feature to compensate for minor dough losses during the preparation of the dough (such as some of the dough sticking to things like the sides of the mixing bowl, the attachments, work surfaces, etc.). For a NY style dough prepared in a stand mixer, I have found that a bowl residue compensation factor of 1.5% is about right. That value usually produces as bit more dough than I want but I simply scale the dough ball weight to the desired value (21.61 ounces in your case) on my digital scale.
Now that I see what you recipe looks like in baker's percent format, you might revisit the amount of oil you are using. An oil value of about 0.31% (I am assuming olive oil but the numbers will be very similar for other oils) is quite low and perhaps can't be readily detected in the finished crust. Some pizza operators who specialize in the NY style frequently omit the oil altogether but other operators like to use something like 13% oil. That makes the dough a bit more extensible and it also provides some flavor to the finished crust.
Peter