There are three things you need to know to determine the amount of flour to use for a particular pizza: the size (diameter) of the pizza (18 inches in your case), the desired thickness (i.e., the thickness factor TF), and the baker's percents for the particular dough recipe you are using.
Since I don't already have a posted Lehmann 18-inch dough recipe for your situation, let us assume that you want to make the 18-inch pizza with a thickness factor of 0.105, which is fairly typical of a Lehmann NY style pizza (you can use 0.10 if you want an even thinner pizza). Let us also assume that you are not changing the baker's percents for the salt (1.75%) and oil (1%) in the basic Lehmann NY style dough recipe, and that you are using 0.50% IDY.
The weight of dough needed in your example is equal to Pi x R x R x TF, where Pi = 3.14 and R is the radius (equal to 18 divided by 2), or
3.14 x 9 x 9 x 0.105 = 26.71 oz.
To determine the amount of flour that you will need, you first have to add together all of the baker's percents for the recipe. In your case, it is 100% (flour), plus 60% (water), plus 1.75% (salt), plus 1% (oil), plus 0.50% (IDY). That comes to 163.25. You next divide 163.25 by 100. That gives you 1.6325. The final step for the flour weight calculation is to divide the weight of dough calculated above, 26.71 oz., by 1.6325. Doing this gives us 16.36. That's the weight of the flour you need for your particular dough formulation. The weights of the remaining ingredients are calculated by multiplying the weight of flour (16.36 oz.) by the respective baker's percents for the remaining ingredients. For example, using your hydration percent of 60, the weight of water you will need is 60% of 16.36, or 9.81 oz. The amount of salt needed is 1.75% of 16.36, or 0.29 oz., and so forth for the rest of the ingredients.
If your baker's percents or thickness factor TF is different from the above example, that's no big deal. Just go through the calculations described above but use your particular values. In the future, you might also try out one of the spreadsheets now available on the forum.