By and large, I think your maiden effort with the Lehmann dough formulation turned out quite well. This leads me to believe that using bread flour may be something you may want to consider in a future effort.
I also think you put your finger on some of the causes of the stiffness in the dough. Looking at your photos, the crevices and cracks in your dough are typical of those often found in doughs that are under-hydrated, that is, don’t contain enough water in relation to the amount of flour. If that was the problem in your case, then the simple solution is to just increase the amount of water. To modify the Lehmann dough formulation to achieve this objective, you can visit the Lehmann dough calculating tool at http://www.pizzamaking.com/dough_calculator.html
and reenter the baker’s percents you last used but change the hydration percent to something above the 60% you used. Even then, you might find that you need more water to get the desired finished dough consistency and feel at the end of the dough making process.
Note also that at this time of year, doughs can also turn out a bit drier because of room temperature and humidity factors. Tom Lehmann discussed this aspect of dough making in a PMQ article at http://www.pmq.com/mag/2006march/lehmann.php.
Remember also that even when one accurately measures out ingredients, there is frequently a need to make minor adjustments in the mixer bowl. BTW, I wouldn’t become too preoccupied with trying to weigh out the lightweight ingredients like salt, yeast and oil (and sugar, if used). I, too, have a special scale for weighing out small quantities of lightweight ingredients but I have discovered that the volume measurements recited in the Lehmann data are quite accurate. Weighing out the flour and water should be sufficient for your purposes.
If you’d like, you can also alter the Lehmann dough formulation to produce a smaller dough ball weight that, when used to make the same size pizza you made, can have a thinner finished crust. The easy way to do this using the Lehmann tool is to use a smaller thickness factor. For example, you might try using 0.095-0.10. I can't promise you that the crust will be less chewy. That is a crust characteristic that is common with the Lehmann NY style. Using a higher protein flour usually increases the chewiness of the crust. But it may not be as noticeable if you use a thinner crust.
Like you, I have difficulties making small amounts of dough using my KitchenAid stand mixer with the C-hook. For this reason, I have been experimenting lately with alternative approaches to prepare my pizza doughs (including Lehmann doughs), which I have discussed at this thread: http://www.pizzamaking.com/forum/index.php/topic,3985.msg33251.html#msg33251.
I am not proposing that you abandon your current approach in favor of the one I have been testing, but there are some simple measures that you can take that I think will improve the hydration and quality of your dough. For example, using sifted flour and the whisk and flat beater attachments of your KitchenAid mixer can improve the handling qualities of the dough, even at high hydration levels. I discovered also that I can dispense with the flat beater attachment and finish the kneading process by hand, in the bowl and on the work surface. If someone doesn't have a stand mixer, the action of the whisk can be replaced by an electric hand mixer operated at low speed.
EDIT (3/22/13): For the Wayback Machine link to the Lehmann PMQ article, see http://web.archive.org/web/20110824144931/http://pmq.com/mag/2006march/lehmann.php